Limbo
Considering all of the time and effort Cobb and Mol put into their world (limbo), it's awfully drab. My first few viewings I accepted the representation of limbo as a kind of "grey area", like we are only seeing an echo of what they built. I suppose I still see it that way, but now I kinda wish they had shown us an ACTUAL "world of their dreams".
Granted, to do that could have been its own movie. In fact I think that has been done before. But stil...