I think Inception might have the same "problem" that Blade Runner has. Although, knowing how much a fan of Blade Runner Nolan is, perhaps it's on purpose.
Blade Runner is ostensibly about loss of humanity, and that Roy Batty, a replicant, can feel and love life more than a human. But if the "twist" of it all is that Deckerd is a replicant himself, it kind of robs the movie of thematic weight for a satirically dark twist.
Throughout Inception, even to the very end, seemed to me to be about the process of learning to let go, of the past and of our fantasies that we think we require to survive to find the joy we have in the here and now. This seemed to be Cobb's arc. But if the intention at the end is that Cobb is still in a dream world, it robs the film of its thematic weight, again, for the dark, pessimistic twist.
Upon the end of the film, I felt the character had completed his arc, and learned to let go of his emotions and fantasies of the past so that he could be mentally healthy and enjoy the reality of today. I never even thought that anything else happened, that feeling upon the ending is what felt natural and right to me. Once I started thinking that in the end Cobb was still dreaming, the film becomes weightless (pun not intended) to me, and merely an exercise in entertainment.