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.
This makes sense, though as a cleric aficionado reading through the first UA with cleric rules made me reflexively wince several times. From a balance point of view it is much more simplified to have all classes using the same general template for progression, and the multiclassing powergamer “dip” concerns are valid for stacking benefits at first level.