Updated rules on seven classes: Bard, Cleric, Druid, Monk, Paladin, Ranger, and Rogue, along with multiple subclasses for each of those classes, new Spells, revisions to existing Spells and Spell Lists, and several revised Feats.
It's a lot to go through, the biggest one yet. Personally, I'm thrilled that they backed off from requiring templates for Druid's Wild Shape, but still feel some type of way they're now limiting your number of known forms. I sort of understand it, but I don't love it.