Still one glaring issue for me... No content to look forward to. Atleast in WoW there was the next Raid, in GW2 it is "man am I gonna look badass" or "man this PvP is fun, in Diablo III it is "Man am I going to do absolutely nothing but run act III again but this time I will do it slightly faster". I guess there really is just nothing to look forward to, I loved Diablo II and I realise essentially it is the same thing but there is a reason games aren't made like that anymore because now for $60 you are expected to bring more than 4 hours of unique gameplay to the table.
This is the biggest issue with the game still I agree. Monster levels and the infernal machine will help a bit but until pvp comes out. The only thing propelling you forward in diablo is the quest for the next piece of gear.
The main reason I still play from time to time is the simple fact that it is something that takes no more than 50% of my concentration allowing me to do other stuff at the same time.