There really needs to be quests again that permanently make your character better. After killing all the final bosses on Diablo 2 you still had to go back and do optional quests. The Den of Evil quest gave you a free skill point to spend, Act 2 had another optional quest that also gave you a free skill point. Then in Act 3 if you found the jade figurine Alkor gave you a potion that permanently increased HP. But with Diablo 3 the optional quests really just grant you some experience and an achievement so they really are just optional.
Diablo 3 would have more longevity if Act I normal had an optional quest that granted you +1 Strength (depending on class) permanently and the same optional quest on Inferno granted you +4 Strength permanently. And just to add more difficulty, make it so you would require a minimum monster power to be eligible to do a power-up quest like I just mentioned. Normal difficulty power-up quests would require no monster power. Then Nightmare would require at least 2 monster power to do a power-up quest, finally Inferno power-up quests would require you to be at least MP 6.
I would expect there to be something like this in the expansion, but it would be nice to have a patch with a new power-up quest for every act. More players would come back to Diablo 3 IMO since it would feel like a mini-expansion just came out.