A quick rundown on interesting changes and nuances in rules glossary I've noticed, since no one else has made a thread dedicated to this topic.

• Ability Check. Skills. "...if a rule refers to a Strength Check (Acrobatics or Athletics), you can add your Proficiency Bonus to the check if you have Acrobatics or Athletics Proficiency". It might be an oversight or a case of cross-checking, but usually acrobatics is associated with dexterity. IMO, the cases when acrobatics is actually useful are pretty rare and there's a significant overlap with athletics, so I'd just fold acrobatics into athletics as one catch-all movement skill and check it with strength or dexterity depending on circumstances.
• Ability Check. Difficulty Class. "The default DC for a check is 15, and it is rarely worth calling for an Ability Check if the DC is as low as 5, unless the potential failure is narratively interesting". I believe this is an indirect answer to a popular question, how often should DM make people roll checks.
• Armor Training. It's been boggling my mind for a while, but I can't yet see the reason for this change of wording/category. Given that wording tends to affect interactions between rules, it usually means something, but I fail to see implications here. Any thoughts?
• D20 Tests. "Whenever a player character rolls a 1 for a d20 Test, that character gains Heroic Inspiration". This deserves a mention, though it's an obvious experiment akni to the previous one with critical success. It's clear that WotC want inspiration mechanic to play a bigger role, but to me, this consolation prize for a failure seems even more awkward than inspiring success. At least an inspiring critical success on a skill check (without warranting an auto-success, mind you) worked as a noncombat analogue of a critical hit.
• Difficult Terrain and Move. "A space is Difficult Terrain for a creature if the space contains any of the following: Creature that isn’t Tiny, Furniture that is Small or larger, Heavy snow, Heavy undergrowth, Ice, Liquid that’s between shin- and waist-deep (any deeper and you need to Swim), Narrow opening that is sized for a creature one Size smaller, Pit or another gap of 2–5 feet, Rubble, Slope of 20 degrees or more". Now you can move through the space of other creatures if they're two sizes larger of smaller than you (stated later in the Move rule). It's also nice that examples clarify a lot.
• Exhausted (Condition). It is now much milder in comparison to how brutal it was before. It simplifies survival scenarios, and makes barbarian berserker subclass frenzy feature (if it makes it to playtest as it is now) actually worth using.
• Fly Speed. The old text stated that if a flying creature is knocked prone or has its speed reduced to 0, it falls. The new text says that you fall only if you're restrained or incapacitated. This is a significant buff to flying creatures.
• Hidden (Condition). "Surprise. If you are Hidden when you roll Initiative, you have Advantage on the roll". Given that one of the points in incapatitated (condition) description is also named "surprised" and is a reverse of that, and that there's no surprised condition that would make you skip the first round in combat, it leads me to suspect that they might be removing the surprised condition altogether. But at this point it's just a speculation.
• Hide (Action). "With the Hide Action, you try to conceal yourself. To do so, you must make a DC 15 Dexterity Check (Stealth) while you’re Heavily Obscured or behind Three-Quarters Cover or Total Cover, and you must be out of any visible enemy’s line of sight; if you can see a creature, you can discern whether it can see you. On a successful check, you are Hidden. Make note of your check’s total, which becomes the DC for a creature to find you with a Wisdom Check (Perception)". Now this is big. A DC15 check is an example of D20 Test rule logic, using a DC15 as a basic difficulty. So now, instead of stealth vs perception contests with every guard, you set yourself a DC for your stealth and go with it until you're found. This streamlines things a lot, though not everyone will like it.
• Influence (Action). I like it that it's getting codified in the PHB, along with the basic table of dispositions and reactions. This reminds me of Torment: Tides of Numenera videogame (which I highly recommend to anyone who likes a really well-written story in a weird, non-standart setting), where you could sway the entire crew of the final boss to your side if you possess the right information and have high social skills. One could argue that you can swing a sword and talk... Well, no. Passively defend yourself, maybe, but it takes quite some focus and control over breathing to actually fight. Shout a short order to your teammate, maybe. But not actually parlay with someone in the thick of combat. I think it's right that Influence takes an action, especially now that there's a reference table saying that you can potentially make creatures stand down if you reason with them. The more stuff is codified in the rules, the less is left to DM's interpretation/mercy, the better. I also expect bards to have something to augment this action in the future.
• Invisible (Condition). It no longer says you're heavily obscured for the purposes of hiding, so RAW you can't hide just by being invisible. But at the same time, it gives all the benefits of Hidden (Condition) without any drawbacks. Essentially, it's an upgraded version of Hidden.
• Jump (Action). Jumping is an action now, which I'm heavily opposed to. In combat, this will change the behavior of players around the fact that getting across a relatively small gap now renders you unable to attack or cast spells for the round, though at least it doesn't expend your movement. I'm fine with jump distance being a result of a check, but jump costing an action - definitely not
• Light (Weapon Property). Finally the plight of dual wielding gets a resolution. No more wasting a bonus action, no more conflicts with class features. However, new problems arise. First, the dual wielder feat lets you change only one weapon to non-light. Which is less than +1 to your average damage. And no one in DnD multiverse is capable of dual wielding two non-light weapons anymore. What I can suggest: let the dual wielder feat let you use two non-light weapons, and let you forgo an off-hand attack to insrease your AC against enemy's first attack by PB, or reduce its damage by an amount rolled with your weapon's damage die.
• Long Rest. Now recovers all of your Hit Dice, not just half, and if it gets interrupted, you may still gain the benefit of a short rest. A straight buff.
• Search (Action) and Study (Action). Frankly, I don't know what to make of these. While it's good that such things become codified in the rules, but making these actions in combat, or spending just 6 seconds to study a book (or a bonus action with one of those game-breaking feats) sounds ludicrous.
• Spellcasting (Bard and Ranger): "Whenever you finish a Long Rest, you can practice your bardic arts and replace any Spell you have prepared for this Class with another Arcane Spell of the same level, abiding by the school restriction above". The nuance here is that the current wording of spellcasting feature for classes that prepare spells says, "you can change your list of prepared spells when you finish a long rest". I believe it means that bard and ranger can switch only one spell on a long rest, which makes sense for classes that used to have fixed lists of spells learned, which they could only change - one spell at a time - on a levelup. Now they can do it (change one spell) on each long rest instead. I suppose that druid, cleric, and wizard will still have the benefit of changing their entire list of prepared spells on a long rest, while warlocks and sorcerers will probably be able to change only one.

Feel free to correct me, discuss, or add anything you've noticed that I didn't see.

The changed Armor Proficiency to Armor Training because you don't add your proficiency bonus to armor.  They address this in the video.

Quote from kamchatmonk >>

A quick rundown on interesting changes and nuances in rules glossary I've noticed, since no one else has made a thread dedicated to this topic.
...

• Armor Training. It's been boggling my mind for a while, but I can't yet see the reason for this change of wording/category. Given that wording tends to affect interactions between rules, it usually means something, but I fail to see implications here. Any thoughts?

...
Feel free to correct me, discuss, or add anything you've noticed that I didn't see.

 
They actually answered this in the Feats video, I think it was.  They changed it to "Armor Training" because "Armor Proficiency" was the only time you were given 'proficiency' in something without it involving your proficiency bonus.  The change to 'Armor Training' is an attempt to clean up their language and ensure that everything which mentions "Proficiency" uses your Proficiency bonus in some way.

EDIT: Golaryn beat me to it.  Running some morning gaming, be with ye on the rest in a bit.

Quote from kamchatmonk >>

• Search (Action) and Study (Action). Frankly, I don't know what to make of these. While it's good that such things become codified in the rules, but making these actions in combat, or spending just 6 seconds to study a book (or a bonus action with one of those game-breaking feats) sounds ludicrous.

 I think it's good to define these actions because it gives a basis on how to study/search that 5e did not give. So it makes it easier for players to know how long it will take to do either of these thing, and it makes it easier for DM's to know how that works. Anyways, one reason why I think they made search an action is because thieves have Second Story Work, so they can do the search action as a bonus action. And searching something is not an investigation check, it's effectively just looking at different scenarios to see what you see, so I think that taking 6 seconds makes sense. That being said, study more closely resembles a typical investigation check, even if it is only investigating one area or object, and I agree with you on the fact that it always takes 6 seconds makes absolutely no sense.

I like the old exhaustion better. It was more complex but the simplification is just -minuses to rolls and makes it less deadly. No one likes to loose a character but if there is no risk it doesn't feel like an accomplishment.

Quote from Pastorbarndog >>

I like the old exhaustion better. It was more complex but the simplification is just -minuses to rolls and makes it less deadly. No one likes to loose a character but if there is no risk it doesn't feel like an accomplishment.

 I agree. I like what does for Berserker, but simply changing Berserker would have been better.

Quote from Pastorbarndog >>

I like the old exhaustion better. It was more complex but the simplification is just -minuses to rolls and makes it less deadly. No one likes to loose a character but if there is no risk it doesn't feel like an accomplishment.

This new design of Exhaustion opens up the possibility to use it more often in the game without it being so quick to automatically kill characters. There can be monsters that cause you to take 1d4 exhaustion under certain effects now (possibly based on recharge abilities). There can be more environmental conditions that cause exhaustion. There can be more subclass features that grant exhaustion. 

There are still risks from taking exhaustion. It just takes more. Which 1D&D will probably provide later on. 

Fly speed: unless it has the hover property or is being magically held aloft, it should still fall to the ground if the movement is brought to zero.

Not a fan of the softened Exhausted rules, either.

Quote from Golaryn >>

The changed Armor Proficiency to Armor Training because you don't add your proficiency bonus to armor.  They address this in the video.

 Oh. My bad. I was watching feats video way after midnight while my brain was halfway to Far Realm. Sorry fot the lack of attention)

Quote from BoringBard >> 

I think it's good to define these actions because it gives a basis on how to study/search that 5e did not give. So it makes it easier for players to know how long it will take to do either of these thing, and it makes it easier for DM's to know how that works. Anyways, one reason why I think they made search an action is because thieves have Second Story Work, so they can do the search action as a bonus action. And searching something is not an investigation check, it's effectively just looking at different scenarios to see what you see, so I think that taking 6 seconds makes sense. That being said, study more closely resembles a typical investigation check, even if it is only investigating one area or object, and I agree with you on the fact that it always takes 6 seconds makes absolutely no sense.

 Like I said, good thing it got codified, but the realization is lacking. Besides, what's the principal differnece between investigating an item and assessing the state of a creature. Both are essentially examination. I'm not even sure these should be separate actions.

Quote from Xukuri >>

Fly speed: unless it has the hover property or is being magically held aloft, it should still fall to the ground if the movement is brought to zero.

Not a fan of the softened Exhausted rules, either.

 That's the old text, and it doesn't work that way in UA anymore. A grappled flying creature remains flying, it doesn't fall to the ground. And apparently knocking flying creatures prone either makes them fall to the ground (if Prone rule wording would say so), or does nothing. This rule's wording needs a rework.

No need for any apologies, I was just attempting to answer the question.

I'm actually a big fan of the new Exhaustion rules.  Old Exhaustion was so drastically punitive you couldn't really get away with inflicting more than twoish levels of it - any more and you were basically removing a character from the game without giving them any recourse.  This new version is a much more gradiated effect, which means DMs can be much more cavalier about inflicting Exhaustion, especially in harsh survival games or even the ever-awesome but extremely dangerous method of having monsters inflict Exhaustion with their abilities.  I think it's a much more useful system this way than the current version, and I'll likely use this method even if they decide to change it back.  Which I dearly hope they don't.

Quote from Yurei1453 >>

I'm actually a big fan of the new Exhaustion rules.  Old Exhaustion was so drastically punitive you couldn't really get away with inflicting more than twoish levels of it - any more and you were basically removing a character from the game without giving them any recourse.  This new version is a much more gradiated effect, which means DMs can be much more cavalier about inflicting Exhaustion, especially in harsh survival games or even the ever-awesome but extremely dangerous method of having monsters inflict Exhaustion with their abilities.  I think it's a much more useful system this way than the current version, and I'll likely use this method even if they decide to change it back.  Which I dearly hope they don't.

 I'd definitely agree, but I could see a few extra detriments being added on to the levels of exhaustion. I'd think some slow reduction of movement speed might be in order (maybe -1 or -2 feet per level of exhaustion).

Just thought about this. The light weapon change does allow for dual wielding monks they can easily get the fighting style with the class grouping.

Quote from Bran >>

Just thought about this. The light weapon change does allow for dual wielding monks they can easily get the fighting style with the class grouping.

 Interesting. But I doubt it. Isn't a part of monk's design. Unless they introduce some kind of alternatives for unarmed combat or staff. Then again, judging how two-weapon fighting has changed, we can expect similar changer to martial arts.

I really like most of these actions being codified. While most of these actions will not be performed in combat without a feat or a special class feature codifying how some of this works can really help new GMs and players a-like. Jump as an action is interesting. I really like the distance that you can go and the Jump spell and Monk's step of the wind definitely get a big boost by this. Since the DC is 10 the minimum distance anyone will ever jump is 10 on a successful check. With the Jump spell this becomes 30 feet, with Monk's step of the wind this is 20 feet (of course that is the horizontal distance). In addition, while it says you can't jump farther than your speed Jump and Step of the wind modify your jump distance, the question would be does it modify your max distance as well? 

Further I am further BAFFLED by the loss of Thief's Use object or even interact with object as a bonus action loss given the fact that Use magic item has been placed under the MAGIC action.

"MAGIC [ACTION] When you take the Magic Action, you cast a Spell that has a casting time of an Action, or you use a Magic Item that requires an Action to be activated. If you cast a Spell that has a casting time of 1 minute or longer, you must take the Magic Action on each turn of that casting, and you must maintain Concentration while you do so. If your Concentration is broken, the Spell fails, but you don’t expend a Spell Slot." 

I know it already worked like this, but that was because "use object" and "use magic object" were different, but sounded very similar. Now using magic items being in the same space as casting spells makes this much easier. This ALSO probably fixes loop holes that people used where if you wanted to use a magic item to cast a spell instead of using the cast a spell action and breaking invisibility since invisibility spell didn't specify using a magic item as a condition for breaking it.

Gaming sets and musical instruments. One thing I am still wondering is why are there multiple instrument or gaming set proficiencies? What is the mechanical difference between having proficiency with a horn and proficiency with a flute? If it wasn't for the fact that you could gain advantage if you had performance AND a musical instrument I would be really wondering why Instrument proficiency existed at all instead of just performance. 

I am also on board with the idea of athletics and acrobatics being a single skill just called Athletics. At this point they both basically do the same stuff, and we NEED more uses for both of them anyway. They codified uses for most of the other skills, but at the same time Codified OUT uses for athletics and acrobatics. Grapple doesn't do it any more. Swimming and climbing is just moving at half speed, or 1/3 speed if the swim or climb is difficult terrain. So far that leaves 2 things left for athletics and 2 things left for acrobatics. Jumping (for both), maintaining balance for acrobatics and lifting for athletics and that is all I can think of.

Quote from Aquilontune >>

Further I am further BAFFLED by the loss of Thief's Use object or even interact with object as a bonus action loss given the fact that Use magic item has been placed under the MAGIC action.

 I believe this has been done to eliminate the potionholic abuse, though I don't really see it as much of a deal. If a PC wants to throw stuff that costs money around, let them.

Gaming sets and musical instruments. One thing I am still wondering is why are there multiple instrument or gaming set proficiencies? What is the mechanical difference between having proficiency with a horn and proficiency with a flute? If it wasn't for the fact that you could gain advantage if you had performance AND a musical instrument I would be really wondering why Instrument proficiency existed at all instead of just performance. 

This weirds me out, too. Basically, performance skill makes you a theatrical actor or a stand-up comedian. Not the most crucial thing in DnD. While common sense says that playing violin and playing bagpipe are very different skills, mechanically there's no difference whatsoever in game. And how does proficiency with musical instruments even work without perform? You can play, but you can't... do it artistically? Do it on public? So yeah, I'm all for folding musical instruments into perform skill.

I like the general move towards codifying actions, but between the way Jump cuts off player movement and the idiosyncrasies of Study/Search, I think the implementation is off. For starters, I think there needs to be a distinction between the combat use of an action and out of combat use, to avoid the weirdness of Studying always taking six seconds or less (to say nothing of Influence). For another, everything is competing with the almighty Attack/Magic action, which means these actions need to be massively impactful to even be considered as it stands. FFG's Star Wars system has a similar combination of "skill checks take an Action" with fast combat as 5e, and it suffers from the same problem.

If I were the developer, I'd make Study and Search bonus actions in combat (with the corresponding expectation that the level of detail will be lower than a thorough non-combat use) and let the associated feats allow for free action use once per round. Jump worked fine as part of a move, but if they feel that having a special action that's performed as part of moving is too potentially confusing, then make it a bonus action too, and give Athlete or Speedster an upgrade to free action once per round, making it expend movement in either case.

Quote from Kaigen_42 >>

I like the general move towards codifying actions, but between the way Jump cuts off player movement and the idiosyncrasies of Study/Search, I think the implementation is off. For starters, I think there needs to be a distinction between the combat use of an action and out of combat use, to avoid the weirdness of Studying always taking six seconds or less (to say nothing of Influence). For another, everything is competing with the almighty Attack/Magic action, which means these actions need to be massively impactful to even be considered as it stands. FFG's Star Wars system has a similar combination of "skill checks take an Action" with fast combat as 5e, and it suffers from the same problem.

If I were the developer, I'd make Study and Search bonus actions in combat (with the corresponding expectation that the level of detail will be lower than a thorough non-combat use) and let the associated feats allow for free action use once per round. Jump worked fine as part of a move, but if they feel that having a special action that's performed as part of moving is too potentially confusing, then make it a bonus action too, and give Athlete or Speedster an upgrade to free action once per round, making it expend movement in either case.

 I wouldn't say the FFG star wars suffers from this. The timing in FFG's Star Wars is technically even more "wishy washy" than DnD. A single "attack action" is technically a series of attacks, for example. And they specifically have structured time vs unstructured time in that system with things in structured time lasting 1 round and outside of structured time lasting 10 minutes. Yes skills are used much less in combat than just shooting, but that is always dependent on how a character is built in that system. It is not uncommon to see people be pretty specialized and since there are multiple "careers" that are not combat focused in that system, when combat does break out they can often times find something better to do besides attack. 

Though I can definitely seeing it be an issue in DnD because classes are all combat classes to some degree. 

Quote from Aquilontune >>

 I wouldn't say the FFG star wars suffers from this. The timing in FFG's Star Wars is technically even more "wishy washy" than DnD. A single "attack action" is technically a series of attacks, for example. And they specifically have structured time vs unstructured time in that system with things in structured time lasting 1 round and outside of structured time lasting 10 minutes. Yes skills are used much less in combat than just shooting, but that is always dependent on how a character is built in that system. It is not uncommon to see people be pretty specialized and since there are multiple "careers" that are not combat focused in that system, when combat does break out they can often times find something better to do besides attack. 

Though I can definitely seeing it be an issue in DnD because classes are all combat classes to some degree. 

 I'm really not concerned with the simulationist aspect of it (i.e. how long it "should" take, based on "real life.") so much as game balance. When is Studying ever going to be worth it compared to attacking? For most monsters, even if a Study check results in the DM handing over the whole stat block, they're not going to learn anything that will make taking the creature down faster than if they had just gone on the offensive, and most DMs are a lot stingier than that when it comes to knowledge checks about monsters. The rules for the Study action itself talks about "call[ing] to mind an important piece of information about it," which implies that all you'll get is a single factoid.

I think the “Strength (Athletics or Acrobatics)” but is good, it indicates they are potentially no longer going to be hard setting each skill to a single ability score and I’m all fort that. I think DMs should be encouraged to to pair various skills with various ability scores when it makes sense to do so.

I dislike the entire Inspiration rule and wish WotC would stop trying to cram it down our collective gullets. Are they being paid off by the Inspiration lobby or something?!? If you have a rule that nobody uses, do you force it on people or simply drop it? I say drop it.

I like the changes to Exhaustion, bravo.

For the Hide action, I basically always treated it that way except for the initial check to see if your hide was successful having a DC. You hid and rolled, and whatever you rolled was the DC for them to find you. I’ma just keep doing it that way because I see no reason not to.

I like having a codified Influence action, I just don’t like the DCs and wish there was more of a sliding scale DC based on conditions and level of hostility.

I agree 100% that jumping shouldn’t be an action. I like the roll for distance, but it should be part of movement.

I love that Long Rests now restore full Hit Dice, I just wish it only restored half Hit Points. The automatic full healing of D&D is something I don’t like.

The Search action was always an action. I’m glad now that they have officially added in the Study action and codified these things and what they’re used for.