Okay, a month later and having considered a lot of different opinions, watched the Deep Dive videos, play tested on my own monk, etc., here is my current, comprehensive take on the base class. Spoiler: it hasn't changed much since my initial reaction, except to get worse:
Level One
Proficiencies: NERF. simple weapons - most of us missed that this is a huge nerf. Martial feats now require you to have proficiency with martial weapons. This locks monks out of all martial feats. No more sentinel, great weapon master, you name it. Massive nerf.
Martial arts: NERF. Yes, it went up by one die (so an average of +1 damage to attacks, woohoo), but you can now only use the martial arts damage die for unarmed attacks, which is a straight nerf at high levels if you want to use a weapon to access weapon masteries. Actually, it's a straight nerf at all levels, because see below. Bonus unarmed strike is unchanged. Dextrous attacks is slightly nerfed because the number of simple weapons has been reduced (no more short swords, specifically).
Unarmored defence: no change.
Weapon Mastery: NERF. How is this a nerf? Because every other martial class, and several non-martial classes (rogue, paladin, ranger) just got better, and monks can barely use this feature, and not without cost. For example, what are the standard monk weapons - spear and quarterstaff? What is the mastery for spear and quarterstaff? Flex. Who doesn't use shields? Monks. I mean, this is the worst mastery anyway, but monks still can't use it. Monks could use daggers, if they want to trade damage for mastery. As well, at high levels the monk's martial arts die exceeds the damage of any weapon the monk can choose, so, again, no mastery for monks.
Level Two
Martial Discipline: no change other than the name. I think all of us expected something here.
Flurry of Blows: no change.
Patient Defence: no change.
Step of the Wind: BUFF. You now get to combine the extra move and disengage instead of choosing between them, for one di point, which takes this from being straight trash to being satisfactory. It shouldn't cost anything other than a bonus action to use, but at least it's no longer an insult compared to what rogues get for free with cunning action.
Unarmored Movement: no change.
Level Three
Deflect Missiles: WASH. In the video, WotC made out like this got a damage buff. I'm not seeing it - two rolls of the martial arts die versus 1 roll plus dex modifier, roll to hit vs. saving throw...eh, that maths out roughly the same. The attack portion of this ability is still an ability that you might use once, if ever, at the cost of a di point. It should be free. The catching missiles part is unchanged and...fine. It's very situational and doesn't come up much - my current monk is level 8 and has used it exactly 0 times.
Level Four
Slow Fall: no change
Ability Score Improvement: NERF. See weapon proficiency above. Though realistically monks are so MAD that they normally can't take many feats anyway, so I guess it's kind of a wash?
Level Five
Extra Attack: no change.
Stunning Strike: NERF. You get now one chance per turn, it's still on a constitution saving throw (so, the easiest saving throw for most opponents), and if it fails, you get nothing. I know the current iteration is criticized for making monks into one trick ponies...but now they are no trick ponies. Come on, WotC - put some effort in and redesign the whole thing. Oh, and now it expires at the beginning of the monk's next turn instead of the end, so the monk doesn't get to benefit from their own stunning strike, which is just adding insult to injury.
Level Six
Empowered Strikes: Wash. Maybe technically a nerf, but few creatures resist force damage.
Level Seven
Evasion: no change.
Heightened Metabolism: BUFF. Finally, at level 7 monks get to not worry as much about having enough discipline points. I mean, unless it's a long fight. Still, this is very good.
Stillness of Mind: NERF. Removed (but effects wrapped into self-restoration at level 10; see below).
Level Nine
Acrobatic Movement: no change.
Level Ten
Self Restoration: NERF/BUFF: Losing immunity to poison damage is a straight nerf. However, being able to remove charmed or frightened as a bonus action rather than an action is a buff, though you have to wait three more levels to get access to it, and being able to remove the charmed condition is very situational. I think it's a definite nerf overall, and a very underwhelming level ten ability.
Level Thirteen
Deflect energy: ?. Monks lose fluency in all languages (more or less) which was often useful outside of combat, depending on the campaign, but by this level is fairly easy for other classes to get. But they gain the ability to deflect spell attacks, which, I dunno, might come up sometime. See deflect missiles, above. Either ability is pretty situational and weak, so I guess this is a wash?
Level Fourteen
Discipline Survivor: no change other than the name.
Level Fifteen
Perfect Discipline: BUFF (but still sucks). The current level 15 ability, Timeless Body, is a straight ribbon, so anything here would be a buff. Perfect discipline finally more or less keeps the monk in discipline points...but at a level where that is seldom an issue anymore. I doubt your monk will ever use it. And if they do need it...it's weirdly designed. Like, if you start your turn with 1,2, or 3 discipline points, you get nothing from it. So...it's better than the nothing monks previously got at this level, but not by much.
Level Eighteen
Superior Defence: BUFF (more or less). Okay, it costs one less discipline point than the current version, but you lose invisibility. That's a nerf. However, it now uses a bonus action rather than an action, which is way better. So, overall, a solid buff, IMO.
Level Twenty
Defy Death: BUFF (but still weak). The previous capstone, which is now the level fifteen ability, was straight garbage, so this is a definite buff compared to that. However, compared to other capstone abilities, this is very unimpressive. More problematically...Way of Long Death monks already get a much more powerful version of this exact ability...but at level 11. So...what is their level 20 capstone ability, now? I don't think WotC thought this one through, at all.
TLDR: Terrible. Monks can no longer take martial feats, get nothing from weapon masteries, putting them way behind all the other martial classes, have their signature ability heavily nerfed, and most of the buffs still aren't that great. While other classes took a big step forward, the monks at best stood still, and went backwards in important, measurable ways. My feedback to WotC will be that this iteration is a disaster for monks and will drop them from being popularly considered the D tier class to a straight F.
