My playtest campaign that started with Dragons of Stormwreck Isle and the first playtest packet is still running, and since it features a Monk I'll provide some feedback here on what stood out to me over the levels as the changes started coming in:
Level 1: Background Feats allowed the Monk player to pick Tough (on a Dragonborn), and it has made a difference. There are a few encounters where they've been a few points shy of going down. This has allowed the Monk to act as a "Tank" of sorts in a party consisting of Monk, Druid, Cleric, and Rogue, but I think it also indicates that the extra hitpoints have been a requirement in order for it to act in this role. (The Monk is being played by a 5-year-old that doesn't understand all the kiting that's been mentioned in this thread)
Level 2: This was where flurry of blows started being used and it was notable that this ran out almost immediately in the Cave chapter. This is also where a father's love for daughter starts kicking in and I let step of the wind do may more than it should out of combat.
Somewhere in this range the light weapon property changes to work as part of the attack action and between a shortsword, nun-chuck, and kicking the Monk is putting out damage that makes the other players notice at this level even while still using d4's for unarmed strikes.
Level 3: Open Hand Monk was selected and I think around this time the Dragonborn gets revised. Not a huge difference as we were using the one from Fizzban's. The character is doing well in terms of damage output but it is level 2 spells that are often making the difference in the important combats.
Mid-level 3 I start applying the first of my homebrew fixes for the Monk as they are awarded a Dragonhide belt (restore ki as action). The class was taking forever to reach UA and I was trying to prevent the typical problems as I've been running the game in a depth crawl style for dungeons that could lead to multiple encounters before short rest.
Later in level 3, we have weapon masteries introduced and the player puts their shortsword and nunchucks (club) to use. This has to change when the light weapon property changes back to bonus action when the "Nick" property is introduced. This was the first point that a bit of disappointment was felt in the direction of things (on my part), the player was either just swinging the nunchuks or shortsword at random and I was dutifully applying Vex or Slow. These were fun though rarely consequential.
Level 4: We've just reached this level. I know it seems slow but play sessions with children rarely reach an hour and leveling slowed once they passed the starter box materials. UA6 comes out and I had to break the news to the player that they can no longer use their shortsword and the Mage Slayer feat that they had chosen at level is no longer available. This didn't go over well, punching slightly harder doesn't matter much to a player whose cool-sounding Feat needs to be retconned. An effect of the playtest process of course but it does feel strange since this a regression from the 2014 version.
From a personal perspective it is also odd that they wouldn't get access to Charger without jumping through hoops as running up and delivering a flying kick is a very Monk thing. This becomes stranger considering that if the player picked Giff or Eladrin they wouldn't be affected by losing access to all these feats. This gives a very tiered experience to playing the class and I guess my biggest take-away from the playtest packet and yes I do understand that changes should be expected when play-testing.
The other big takeaway is the rather odd changes to the Open Hand Monk (CON save to prevent reaction) paired with the surprising improvements to the Four Elements Monk, I'm thinking that I might pivot the player to embracing their Draconic nature and playtest the new Four Elements Monk instead. I'm also letting keep the short sword.