Our DM just discovered the Sentinel feat and immediately thought it was overpowered.  We convinced him to keep it mostly as is, but we are now playing with a House Rule as follows;

We modify the first benefit to read (Added text in bold)

Whenever you hit a creature no more than 1 size larger than you with an opportunity attack, its speed drops to 0 for the rest of the turn. This stops any movement they may have been taking.

This means a gnome can not use Sentinel to stop a Giant.  

It's not overpowered. It just isn't. Plenty of creatures have ranged attacks, and you have to hit it with a melee attack, so you could still be close enough for it to attack you. 

Not that hard to ensure an AoO get's hit.  Advantage is easy to get and a bless can really do numbers.  Bad guys have a hard time retreating from a Sentinel with Bless and Advantage.

Eh. This fix just seems mean to small PCs. Besides, what's so broken about stopping a huge creature when they usually have melee reach greater than 5 feet?

Say you are a human being.  Fighting a Leviathon.   Under water.

You swim up to it and stab it. With a +3 Shortsword.  Being high enough level you manage to take it down to 1/2 hitpoints.  It decides it wants to escape.  With a swim speed of 120 ft.  While you have no swim speed.

But you think your short sword, along with the feat Sentinel alone can stop this Gargantuan monster that weighs tons and is more than 20 ft wide at it's thinnest point.

Does not pass the smell test to me.  Sentinel is a great feat.  But it alone should not be able to do this.  I can see you stopping creatures your own size or even 1 size larger.  But a man that is not allowed to Grapple a creature more than one size larger than him should also should not be able to stop something cold with a single feat and a succesfull attack.

As for small vs medium, there are a lot of advantages to being small (among other things the ability to ride a medium sized creature into a dungeon).  But just as their are advantages, there are disadvantages as well. 

The slightly weakening of one of the best feats is not unreasonable.

Quote from Mog_Dracov >>

Say you are a human being.  Fighting a Leviathon.   Under water.

You swim up to it and stab it. With a +3 Shortsword.  Being high enough level you manage to take it down to 1/2 hitpoints.  It decides it wants to escape.  With a swim speed of 120 ft.  While you have no swim speed.

But you think your short sword, along with the feat Sentinel alone can stop this Gargantuan monster that weighs tons and is more than 20 ft wide at it's thinnest point.

Does not pass the smell test to me.  Sentinel is a great feat.  But it alone should not be able to do this.  I can see you stopping creatures your own size or even 1 size larger.  But a man that is not allowed to Grapple a creature more than one size larger than him should also should not be able to stop something cold with a single feat and a succesfull attack.

 

As for small vs medium, there are a lot of advantages to being small (among other things the ability to ride a medium sized creature into a dungeon).  But just as their are advantages, there are disadvantages as well. 

The slightly weakening of one of the best feats is not unreasonable.

 
Yeah I mean it's not a huge change, but in this scenario said leviathan could spend the next turn shoving the player then retreating. Or better yet, grapple/swallow them so that it never activates. My reason for saying otherwise is just that this player has already invested a feat, and acquired a powerful magic item, so they should be able to do extraordinary feats. ;)

At high levels, players already have access to plenty of spells/effects that can already stop a creature from moving. I get your point, it is a bit of absurd to think about a puny human stopping a leviathan, but a lot of things characters can do, especially at higher levels, are crazy. Even at first level, characters can use spells like entangle and ensnaring strike to restrain creatures. The creature does still have to make a save, and in the case of ensnaring strike, a Large or larger creature has advantage. By the time characters would be fighting a gargantuan creature, they likely have access to at least 5th level spells, which gives them access to many ways to prevent a creature from moving. By that point, plenty of other features can also help prevent movement, like the Monk's Stunning Strike. They might even have access to power word stun, which doesn't even force the creature to make a save. It does seem a bit crazy to have this happen as a reaction, but this feature is one of the main reasons for taking the feat. According to the PHB, "A feat represents a talent or an area of expertise that gives a character special capabilities. It embodies training, experience, and abilities beyond what a class provides." I think feats are intended to have extraordinary potential and give players chances to do crazy things.

With that said, I agree that your point about a character stopping a gargantuan creature is crazy. I would recommend giving creatures larger than you a chance to make a save against the effect, or make the player make some kind of check, instead of just saying it's not going to happen. Even if the creature is almost guaranteed to make the save, even a minuscule chance of stopping a small creature is exciting. Maybe the DM could also make rulings based on the situation. If everything's stacked against you, like in the leviathan example above, the DM could say that the effect doesn't work, or force the creature to make a save. If you really want to make Sentinel less powerful, you could at least give the player a chance. I just think that having the chance of stopping massive creatures in their tracks is one of my favorite parts of playing a Sentinel.

Edit: Fixed tooltips

If Sentinel is never allowed to halt an enemy's movement, what is the purpose of the feat?

The intent of Sentinel is to allow a melee-focused character to apply that melee ability to foes that do not otherwise wish to be engaged in melee.  The character gives up several other possible options to acquire Sentinel; denying them the chief benefit of the feat is frustrating.  

The last time this came up - because it comes up a lot - the suggestion I had was to apply an 'Unstoppable' trait to specific monsters deemed too momentous or powerful for a Sentinel character to stop.  This trait allows the creature to ignore nonmagical reductions to its movement speed, a'la from Sentinel or grappling.  Rather than saying "Sentinel will never work the way you want it to work so don't bother taking it", consider applying a trait to specific monsters, instead.  Having a capability you invested in be denied by a big setpiece boss only amplifies the "OH SHIT" factor of such a fight, while saying "Sentinel only works on things a Commoner could reasonably be expected to successfully grapple" just makes the feat frustrating and unfun for the player.  At that point, simply ban it from the table outright and have done with it.

There are lots of ways of avoiding opportunity attacks that sentinel doesn't bypass -- effects that remove the ability to take reactions, shoves so the fighter isn't in range, invisibility/darkness/etc so the fighter can't see the target, actions or legendary actions that simply specify not drawing opportunity attacks, ...

All the points made here are good, but they seem to ignore the issue I am talking about.

1) I want Sentinel to work and work well.  But there should be clear limitations to it.

2) Most physical things you can take by 5th level that stop movement have some kind of size limit.  Those that don't have a strength limit.

3) The question is this:

Should someone with just skill (no magic) be able to stop a creature of any size?

That seems ridiculous to me.  Personally, I don't think I could stop people like The Rock, let alone an ogre - no matter HOW much I trained.    Yet I still think the feat should be allowed to stop Large creatures.

But Huge creatures?   or worse, Gargantuan?

You honestly want to tell me that you think that even a professional could stop a charging elephant with a single hit of a sword - without killing it?  Elephants are larger than moose, yet Moose routinely challenge cars and WIN. You hit a moose with a car and the moose is more likely to walk away than the car is to drive away.

Quote from Mog_Dracov >>

Should someone with just skill (no magic) be able to stop a creature of any size?

 Sure? Open Hand Technique has no size limit.

It does make sense to make it harder if you're stopping a creature of a larger size. However, I think it should be possible to do, even if it is more difficult. Maybe make the attacker make a Strength check with a DC based on size, or target make a Strength save with advantage if they're Large or larger (as in ensnaring strike). I do feel that one of the main parts of Sentinel is the ability to stop creatures regardless of size, but if there's a limit, it should still be possible.

If your game feels more fun and immersive with Sentinel reduced to stopping random bandits from fleeing, then go for it.  It's a common houserule.  Simply let your players know ahead of time, and if they still sound keen to take the feat anyways, perhaps allow them to encounter a few more Medium or smaller threats that their investment works on.

Ever stubbed your toe? stopped you real quick right?

that's how a gnome can stop a giant with sentinel. it doesn't have to be a brute force thing as sometimes even small wounds can cause more pain than you realize (papercuts anyone?)

Some commentators seem to be using reductio ad absurdum to make their point...that any exception to Sentinel is equivalent to never letting the PC use it, and the feat becomes worthless The RAW definitely doesn’t have a size or circumstance limitation on reducing the movement to zero when landing an opportunity attack. But since when do any of us only play by RAW in all cases, with no exceptions ever? Respecting the rules is a foundation, but we don’t need to be slaves to them when they lead to an absurd result...the game designers didn’t foresee every possible scenario. Personally, I’d only consider a limited exception for gargantuan creatures, and maybe in unusual cases like a huge+ target making a charge or flyby attack where there is too.much momentum to stop even if it wanted to. Honestly, how often do you fight a gargantuan creature, and would a limited and reasonable exception really render the feat useless and cheat the player? I’ve seen my players get a lot of benefit out of Sentinel without needing to use it to stop a charging terrasque cold in its tracks.

The Sentinel Feat already has several limiting factors.

First, you're either giving up an Ability Score Increase, or the chance to take a different feat, in order to choose this one specific one, which has a very specific purpose (trying to force an enemy to stay and fight you). You're losing a general performance increase, or another specialisation, in order to take this one.

Second, you need to use your reaction to attempt to do it; if you've already used your reaction for the turn, you can't use this ability of Sentinel as well.

Third, to restrict a creature's movement you have to actually hit it with your reaction attack; it's not a reaction to definitely reduce it to zero movement, it's a reaction for the chance to do-so. Higher challenge rating creatures tend to be harder to hit.

Fourth, you have to be able to make the reaction attack in the first place (not just have your reaction handy), which you can't do if the creature can move you away, if it knocks you Prone you'll have disadvantage, and a bunch of other things that harder creatures can do to make it harder or impossible to use this ability.

I think it's also worth remembering that the Sentinel Feat doesn't necessarily represent you robbing the creature of the ability to move, but rather that you are presenting yourself (and/or your allies alongside you) as too great a threat for it ignore or turn its back upon. Reducing its move to 0 is just how it works mechanically.

Personally I think that that's plenty; if you do end up in a situation where it's clearly ridiculous for a creature to stay and fight your Sentinel rather than doing something else, then your DM as always remains free to rule differently, maybe using its reaction or a legendary action so the player is still getting something for their ability even if it still gets to move away.

Lastly, as others have pointed out, there are other ways to restrict an enemy's movement that don't care what size the target(s) are, so there's no real precedent for this to be any different.