Finally, we get our hands on the One D&D version of the Monk. And...
Ok, there are, in fact, a lot of changes, but far fewer than what I had expected. I'll confess a little worry that the approach here has been too conservative for what the Monk really deserves. Now, admittedly, I also think that my experience of playing and seeing monks played has shown them to be a fun and powerful class, regardless of what theorycrafters say, but given that their math is not deeply flawed, I also think the class was due for some kind of buff. And again, there is a bit of one, but... not as much as I expected.
Let's get into the changes.
First, something I expected to change that didn't - the Monk still uses a d8 hit die. I cannot understand why it shouldn't get a d10, given that it's one of three obligate melee classes, and the Ranger, which is most traditionally ranged, gets one.
Actually, another funny thing is that Monks get a Spear, rather than a Quarterstaff, as starting equipment. Now, other than its damage type, a Spear is actually strictly better than a Quarterstaff (though if you're fighting skeletons, you'd prefer the staff.) All that said, though, a Monk might actually be best off dual-wielding daggers and making use of the Nick property.
Speaking of, Monks get Weapon Mastery. With their limitation to Simple Weapons, the options for these masteries is a bit limited, but I think Handaxe/Dagger or Handaxe/Light Hammer is a very solid load-out, which will give you three attacks per turn (though only adding Dex to the first and last).
Monks will get two weapon masteries, and unlike Barbarians or Fighters, they never get more (though I doubt this will be much of a problem). You can swap these out on a Long Rest (so if you get a +2 Sickle, you can grab that property).
Martial Arts is mostly the same, but the damage dice are A: upgraded, so you start with a d6 and go up to a d12 by tier 4 and B: only apply to Unarmed Strikes, so your Daggers won't be doing more damage when you hit levels 5, 11, and 17.
Also, Monks are only proficient with Simple Weapons, so "Monk Weapons" are now just Simple Weapons, but the Greatclub doesn't let you use Dex for it because of its Two-Handed property.
Ki Points have been renamed to Discipline Points in the new "Martial Discipline" feature, and Step of the Wind has been buffed, giving both the Dash and Disengage effects (as well as the doubled jump distance) with the same use.
Deflect Missiles now refers not to ranged weapon attacks, but ranged attacks that deal bludgeoning, piercing, or slashing damage. If you reduce this to zero, you can redirect the attack to any creature you see within 60 feet of yourself that isn't behind total cover, and rather than making an attack roll against them, they make a Dex save (spending one Discipline Point to do so). The damage has also changed to two rolls of your Martial Arts die, rather than treating it as a monk weapon attack. You also no longer need a hand free to catch the projectile.
Rather than letting your "unarmed strikes count as magical for the purpose of overcoming resistances and immunities," Empowered Strikes now simply let you choose to deal Force damage instead of bludgeoning (or whatever). This seems consistent with the phasing out of distinguishing magical versus nonmagical bludgeoning, piercing, and slashing.
At level 7, there's a brand-new feature called Heightened Metabolism, which lets you get a Short Rest once per day if you spend a mere minute resting. So: even if they're trying to remove the reliance on short rests for most classes, this one they've given a way to almost trivially ensure that they get at least one over the course of a day. Notably this does not merely recharge Discipline Points, but gives all the other benefits of a short rest.
Back at level 5, Stunning Strike has gotten a rather big nerf in that you can only use it once per turn, and the stun ends at the start, rather than the end of your next turn. Now, I'm actually in favor of nerfing this feature, but only as payment for other buffs to the class - we'll talk in the analysis at the end here about whether I think the Monk has gotten sufficient buffs (spoilers, I don't think it has).
At 10th level, you get Self-Restoration, which combines Timeless Body, Purity of Body, and Stillness of Mind, effectively. First off, you can use a Bonus Action (rather than an Action) to end the Charmed, Frightened, or Poisoned conditions on you, and you also don't get Exhaustion from forgoing food or drink.
Notably, this removes the full immunity to poison damage and the poisoned condition (which I think could be part of an effort to rehabilitate Poison as a thing in the game.) The lack of mention of Disease is also part of a phasing-out of Diseases as a thing in the game (which were never well-defined) in favor of using Poison and its condition to function as that effect. I still think we need some clarification on how this works if you are, say, under the effects of Dominate Person - can you use that bonus action if your puppet-master isn't directing you to?
Deflect Energy, at level 13, expands your Deflect Missiles to include all ranged attacks, regardless of damage type. Given the way that spellcaster NPCs have been redesigned lately, this could actually be huge, as you'll be able to mitigate (and even redirect) the Arcane Blasts of evil Wizards or the Wildfire attacks of an Archdruid - basically the bread-and-butter damage abilities of spellcasters are now yours to punch back.
Disciplined Survivor is just a rename of Diamond Soul.
Empty Body has been replaced by Superior Defense, which lets you spend 3 discipline points as a bonus action to get resistance to all damage but Force for 1 minute (or until you're incapacitated). This is one point cheaper than the old invisibility but, you know, you don't become invisible. Also, the ability to go on solo Astral Projection trips for 8 points is gone, which I think is utter bullshit.
Perfect Self is now Perfect Discipline, and has moved down to level 15.
At 20th level, you now get Defy Death. If you drop to 0 HP, you can spend 4 Discipline Points and roll four martial arts dice (so 4d12 at this level) and add them together, setting your HP to that number instead (notably, this doesn't count as healing, so stuff like Chill Touch won't prevent it). The cost of this feature increases by 2 Discipline Points each time you use it, but resets on a Short or Long Rest.
Now, there are also a bunch of subclasses to review (though the Warrior of Mercy is as of yet unchanged from Tasha's Way of Mercy, so that isn't in the document) and we certainly aren't getting the full picture without those.
However, I do think that some of the problems the Monk faces are sort of shockingly not really handled here. The Monk is still spread pretty thin on important ability scores, and still has low HP for a class intended to be in the thick of melee. Without any damage reduction (apart from Deflect Missiles, which hardly counts) until very high level (18th for Superior Defense) the Monk is also going to struggle to have good AC (without something like Bracers of Defense or Rings/Cloaks of Protection).
The nerfing of Stunning Strike will have a slightly beneficial side effect by forcing Monks to be more conservative with DP (Discipline Points,) but that's at best a consolation prize.
Now, Weapon Masteries could certainly help the Monk, but not to any greater extent than other melee classes - far from being a special Warrior feature, it now seems like every class that uses weapons is going to get them. Indeed, I almost wonder if they're pulling back on these class groups entirely, which is a shame, because I kind of wanted the Monk to get something that only the Barbarian and Fighter would also get.
Again, as masters of bodily discipline, I cannot comprehend why the Monk shouldn't have at least a d10 hit die if not a d12. The whole point of being a Monk is to be as much of an indestructible force on the battlefield as a heavily-clad knight - if hit dice are meant to represent toughness, surely a Monk's only competition in that regard should be a Barbarian.
Now, damage-wise, the only real buff is to the Martial Arts die, (well, ok, also Weapon Masteries, which can sometimes be a damage buff,) but much as I think Flex is a terrible mastery as it only raises average damage by just 1 per hit, the exact same tiny buff is what they've given to the Monk.
Essentially, in stark contrast to my complaints about the Warlock, which were that they went too far in changing the class, for the Monk I think they took too conservative an approach.