The biggies are obvious. There's the arena combat, which was shelved prior to release and refocused as future free downloadable content. The collectible Rune system transformed into a simple set of unlockable power modifiers.
But then there are the smaller features, the ones not as memorable but no less nifty. Cinematic in-game cutscenes would have been brilliant. Travel powers used to traverse obstacles would have changed the game considerably. The environment destruction is still there, but more cosmetic than anything affecting gameplay.
Diablo III could have been a much different game. At one point or another, it was. Would it have been a better game?
Diablo III: Missing Content #1 [YouTube]