Perhaps I feel the same way. Act 3 is missing something, but I don't really know what it is. It feels more like something I have to do, instead of wanting to do.
I can't really find a root problem, but I can think of possible explanations while Act 3 feels somewhat lackluster:
1) The area is very partitioned. It feels like every house has its own mini-quest. And there probably will be a cellar that is bigger than the house itself. An all-encompassing adventure is absent somehow.
2) It's a city. With houses. And some houses have very big cellars. Which makes the environment rather ... mundane. The thrill of exploring strange new environments seems to be mostly absent in act 3.
3) You reach level 12 at the start of act 3, you send two thirds of your gold to camp because it weighs too much, and you have far too many magical trinkets. Incentives for self-improvement have dwindled significantly in the last act.
