The new playtest is expected to be the largest one yet. It will revisit many classes, include multiple subclasses for every class, and expand on the weapon mastery system introduced in the prior Playtest. WotC says that they are moving toward refining content rather than introducing new ideas at this point.
I'm sure people are going to find all kind of balance issues in the new stuff over time, but it feels like the difference between the worst classes and the best classes is getting smaller (with the exception of wizards).
Overall I'm underwhelmed. They seem to be reverting the most interesing changes from the previous UA, ones which restore health from the game, and not changing the problems, like Monk, enough. (Heighten Materialism looks like an impactful fix, though.) Will have to see after the actual playtest.
I like the changes to Paladin and Rogue most, but Cunning Strike feels like it needs more rounds of playtesting for the idea, and time is something they don't have, so I'm 50/50 on them just removing it.
I'm very excited about Cunning Strike, but it feels like it becomes available way to late for a feature that is designed to be the core feature of a class. It starts at 5th with enhancements at 11th, 13th, and 14th. A core feature like that really needs to kick in sooner.