D4 is one of my most looked forward to games.
First, I want native coop. LMS mod proved that is a winner.
Second, I want the original Cyberdemon back in all it's id tech5 glory.
More wide open battles against multiple enemies (thinking coop again).
Darker than D3
The 2 levels in D3 and D3XP that were totally lights out were awesome.
Much better melee with animations (L4D spoiled me)
A great boss battle at the end that isn't confined to one small area where you knew exactly where and when baddies would spawn, along with an awesome death animation for the boss that does not involve a cut scene. I hated that, as well as the cut at the end of XP.
That's all..for now. Did I mention I really want this game?