I haven't implemented this yet, but I'm really leaning towards it and I'm looking for your feedback.

Instead of the standard critical hit/miss effects (yes, i know there's no critical miss in 5e, but we like it anyway so we kept it), I'd like to change it to effects instead of damage/dropping a weapon type thing.

For critical hits, I'd either destroy a body part (a dragon's wing gets ripped off, an arm gets crippled, an eye gets gouged out, etc) including extra damage but not letting it get super crazy (like with a paladin going nova and ruining a boss battle by finishing it too fast), or maybe just do a random stat and do a significant penalty to it (-4 or -6? depending on damage?), I'm not sure yet.

And for critical miss, something like dropping you to the bottom of initiative, losing extra attacks for 2ish rounds, cutting mobility in half for 2ish rounds, that type of thing.

I know, this is super vague, but I'd really like your feedback before presenting it to the players. I think I'd setup a random roll table just like for loot or whatever, and roll off that whenever a 1 or 20 happens during an attack.  I really like the idea of stopping crazy amounts of damage in 1 round or attack, and having interesting penalties happen for rolling 1s without crippling the players too.

I could envision a list of "good" and "bad" which you could dice roll off of in the event of 20 or 1.  Different bonuses/buffs or pitfalls.  Would definitely spark interest when one of the two events occurred.

Did you have a semi-listing of what you had in mind?

We have implemented a simple system for critical hits and misses. A critical hit gives the character max normal damage plus another roll of their damage dice, then any damage additions are added. Reason: It was very anti-climatic for a character to roll a natural 20 and then the minimum damage. Critical misses allows the TARGET of the missed/fumbled MELEE attack to make a reaction/opportuntiy attack on the attacker who failed. Reason: The natural 1, complete failure, should have more of a consequence on melee combat and this is a simple and quick method that doesn't slow things overly.

I think for rolling 1d6

str - a devastating blow to the abdomen (core) giving a -4 to strength for the rest of combat (try doing anything after being hit hard in the stomach)

dex - a devastating blow to the back/neck giving a -4 to dex for the rest of combat (reducing reaction time and fine motor control)

con - a crushing blow to the chest giving a -4 to con for the rest of combat (treating HP as  the ability to turn harsh hits into glancing blows, endurance, etc)

int - a striking blow to the head giving a -4 to int for the rest of combat (giving tunnel vision, triggering fight or flight impeding critical thought)

wis - a striking blow to the head giving a -4 to wis for the rest of combat (giving tunnel vision, triggering fight or flight impeding critical thought)

cha - a childish taunt followed by a vicious strike that any amateur should have been prepared for, giving a -4 to cha for the rest of the fight (looking foolish reduces force of personality)

maybe make it a 1d8 roll, with 7 and 8 being the player's choice since they DID roll a crit.
int/wis/cha penalties definitely could stop a character from casting spells of certain levels
con definitely reduces max HP

Rolling a 1 could provoke an opportunity attack that gives a similar penalty (probably only -2 instead of -4), for the same reasons (but no extra damage probably?).  I really don't like the idea of penalizing and pissing off the players to the same amount... I want them to feel epic, like heroes. 

And when monsters roll a crit against players, they would have the same effects against players.  Probably cured after a short rest.  Unless a long rest makes more sense to you guys?

My DM uses Crit Hit & Crit Miss charts using percentile dice. Basically if a PC or NPC (friendly or enemy) rolls a crit hit there is an additional effect beyond double-damage-dice. On a miss the other percentile chart has some pretty annoying stuff on it too.

Back in the day we used to use a simple Percentage chart.  The chart ranged from double damage to instance kill along with a wide range of effects causing the target to be slowed (leg hits etc) and knocking the target prone or giving negatives to their AC to to hit for the next turn.

It was pretty cool and I've often thought of casting a raise dead spell on this old sheet. 

I've been thinking of having a more complex critical hit/miss system. I think I'll go for making a table of critical hit effects, and another one for critical misses. Because I want to add cool effects (like dismemberment, hitting an enemy into another enemy, beheading for critical hits and things like throwing your weapon, rolling to hit an ally, hitting a crit against yourself, for critical misses) but not disrupt the balance too much I'm thinking of choosing a method to "confirm" crits/misses. If you confirm it you roll on the corresponding table, if you don't you get the normal crit/miss effect.

Some ways I've considered after reading other editions and comments, depending on how often you want this to happen are:

• Rolling to hit again, confirming a crit with if the new roll would hit under normal circumstances, or confirming a miss if it would miss too.
• Rolling to hit again, confirming a crit with a 20, or a miss with a 1.
• Only allowing this special crits/misses on rolls with advantage or disadvantage. If a roll with advantage results in a normal hit and a crit or two crits, it is a confirmed crit. If a roll with disadvantage misses normally and on a 1, or with two 1s, it is a confirmed miss.
• Same as above but only counting double crits and double misses (two 20s or two 1s).

Most of my critical hit/critical miss stuff comes in with the narration. If one of my players scores a critical hit, I describe them doing something really cool. My problem with random tables are that they often do not take into account what is currently going on in the story. It would be incredibly difficult for a Ranger to dismember an assailant with an arrow, but it would be easy for them to hit the person right in the eye.

For critical misses, I again like to narrate them failing epically. If another friendly character is next to them, sometimes they will accidentally hit them. Sometimes their weapon will become embedded in the floor, and they'll need to use a bonus action (and a STR check) to remove it. I like to vary things. One time, my Fighter kept on rolling Nat 1's during a combat, so I had to get really creative.

Quote from VladTheImploded >>

I haven't implemented this yet, but I'm really leaning towards it and I'm looking for your feedback.

Instead of the standard critical hit/miss effects (yes, i know there's no critical miss in 5e, but we like it anyway so we kept it), I'd like to change it to effects instead of damage/dropping a weapon type thing.

 

For critical hits, I'd either destroy a body part (a dragon's wing gets ripped off, an arm gets crippled, an eye gets gouged out, etc) including extra damage but not letting it get super crazy (like with a paladin going nova and ruining a boss battle by finishing it too fast), or maybe just do a random stat and do a significant penalty to it (-4 or -6? depending on damage?), I'm not sure yet.

 

And for critical miss, something like dropping you to the bottom of initiative, losing extra attacks for 2ish rounds, cutting mobility in half for 2ish rounds, that type of thing.

 

I know, this is super vague, but I'd really like your feedback before presenting it to the players. I think I'd setup a random roll table just like for loot or whatever, and roll off that whenever a 1 or 20 happens during an attack.  I really like the idea of stopping crazy amounts of damage in 1 round or attack, and having interesting penalties happen for rolling 1s without crippling the players too.

Blinded in one eye: You have disadvantage to Intelligence (Investigation) checks, Wisdom (Insight) checks, Wisdom (Perception) checks, and Initiative checks that rely on your vision. Given long enough to adapt to having one eye (usually at least three in-game months), this disadvantage goes away. 

Game leg: Your movement speed, burrow speed, climb speed, and swim speed (as appropriate) are reduced by 10 feet, to a minimum of 5 feet, and you have disadvantage on any Dexterity checks, Strength checks, or Constitution checks related to movement or actions that rely on your legs. You may suffer this injury more than once, in which case its effects on your movement speed stack. If the creature hit has a fly speed, roll a d10. On a roll of 1-10, this injury affects a wing instead: their fly speed is reduced by 10 feet, to a minimum of 5 feet, and they have disadvantage on any Dexterity checks, Strength checks, or Constitution checks related to movement or actions that rely on their wings. On a roll of 11-20, the injury affects a leg as normal. 

Quote from DungeonMaya >>

It would be incredibly difficult for a Ranger to dismember an assailant with an arrow, but it would be easy for them to hit the person right in the eye.

I have been using "Good Hits and Bad Misses" since I started playing.  Page 34, check it out. Dragon #39: Good Hits & Bad Misses.  Once you get into the specific damage types, they are all triple damage effects.  Paizo updated these charts to 3.xE in their Dragon Compendium applying DC values for particular ability saves that can be done to avoid some of the worst damage effects.  I've subsequently applied these tweaks to the original tables, dropped the DCs to 5E equivalents and have been enjoying the glee in my player's eyes as they slice/smash/pierce things with great effect.

Keep in mind though, 20's always being a critical is bad news for PCs, as a single PC will likely be subjected to dozens of critical hits over their career, a career likely shortened with auto-criticals.  One thing 3.xE got right was the confirmation roll on Critical Hits; mostly because I had been using this mechanic back in 2E and still use it.  I have my PCs confirm by rolling a d30 (applying the same bonuses to hit) giving them a higher likelihood of confirming a critical, they are the heroes after all.  Adversaries confirm with a d20, reducing the chances the PC is going to get a leg severed by a lucky shot from a great-axe wielding orc.  

As far as damaging a specific part of the body goes I add ac to hit and or keep in mind an amount of damage for the part of the body. As a example if a player was grappled by a giant tentacle of a monster like a kraken it would be dependent of the amount of damage the tentacle takes not ac.

The problem with messing too much with critical hits/misses is that it unbalances the game.

There are basically 2 types of attacks: The ones you can hit/miss; The ones the enemy does a save and takes full damage or half or nada; 

Both are balanced so that players that attack with a melee weapon, or cast spells that require saves do roughly the same amount of damage over time (considering resources spent and all that, like superiority dices, spell slots, etc).

If you change the damage output and the effects for critical hits/misses, in order to keep the game balanced, would have to compute the average effect your new rule would cause and then create new rules for spell saves too.

For this reason, my advice is, don't change critical hits/misses by yourself, look for UA or DMG variant rules, because those are made taking all the game balance into account.

Otherwise your casters would become overpowered or underpowered.

Thanks for the feedback and suggestions everyone.

The one thing I'd recommend to keep in mind is that rules like this often hinder the players more often than not. Most NPCs are meant to be encountered once and then never seen again, sometimes you'll see them several times, but rarely do you fight an NPC meant to have the story longevity of a PC. Thus, these effects make an already doomed NPC go down a bit faster, but stick around with a PC and hinder them for a long time. Keep that in mind when balancing effects and how easy/difficult they are to treat.

If you can find a copy of it, Try looking in the 2nd Edition Player's Options, Combat & Tactics, Chapter 6.  It has a nice in depth  Critical hit rules and charts for various weapon types and creature sizes.  Most of it transfers to 5e rather nicely without much manipulation.  I must warn, some of it can turn rather lethal for players or creatures, but can add some flair and fun to combat.

The fighter was going up against a small village of tribal warriors, and one of them rolled a 1.  DM ruling, the poor unfortunate hit one of his allies.  Next round, he rolled another 1.  The PC just stepped back and enjoyed the show. 

Quote from Maltfoam >>

We have implemented a simple system for critical hits and misses. A critical hit gives the character max normal damage plus another roll of their damage dice, then any damage additions are added. Reason: It was very anti-climatic for a character to roll a natural 20 and then the minimum damage. Critical misses allows the TARGET of the missed/fumbled MELEE attack to make a reaction/opportuntiy attack on the attacker who failed. Reason: The natural 1, complete failure, should have more of a consequence on melee combat and this is a simple and quick method that doesn't slow things overly.

Quote from Kremdes >>

Quote from Maltfoam >>

We have implemented a simple system for critical hits and misses. A critical hit gives the character max normal damage plus another roll of their damage dice, then any damage additions are added. Reason: It was very anti-climatic for a character to roll a natural 20 and then the minimum damage. Critical misses allows the TARGET of the missed/fumbled MELEE attack to make a reaction/opportuntiy attack on the attacker who failed. Reason: The natural 1, complete failure, should have more of a consequence on melee combat and this is a simple and quick method that doesn't slow things overly.

 I really like this!