Crawford says it’s the biggest yet, with 7 classes (Experts: Ranger, Rogue, Bard; Priests: Cleric, Paladin, and Druid; and Monk confirmed), spells and weapon mastery tweaks, capstones back at lvl20 (epic boons will be pushed to another UA), and subclass progression reverted to the 2014 cadence after lvl3. Notably, rogues are seemingly getting another feature at lvl5 to make up for the fact that they get very nothing from their subclass between 3 and 9.
In order for subclasses to feel impactful and varied, they do need to change how you play pretty significantly…but that means they need to have a pretty hefty set of abilities right when you first get the subclass. I don’t think it’s a coincidence that the classes that are the worst offenders as “most abused multiclass dips in 5e” all get their subclasses at levels 1 or 2.