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.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.
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.