Between Loot 2.0, Clans, and ladder system? I wish I could pick all three but alas, I guess I must choose the clans. With the decimation of the AH, I'll definitely need to find me some people to give away all the awesome items I'm going to finding in sanctuary!
The most exciting thing I've read from DFans regarding XPac so far are the Nephalem Rifts & Adventure mode. Although not much logistically has been uncovered outside of the maps a small bits of information (strings, etc.), the idea of them sound exciting. Can't wait to here more