Now to the nitty-gritty of it. I think Christopher Nolan is, at heart, trying to be a creative person when he's really a technical guy. He knows what works with audiences, but only in the strictest commercial sense of the word "works". He operates in such a way that the widest range of people (not just the lowest common denominator or elitists) can enjoy his films and think they are great. Does that make them actually great? From my personal standpoint, no. From an objective standpoint taking in the history of film and narratives in general, not quite.
What Christopher Nolan's films lack in general is a feeling of true inspiration. Everything Nolan does is achingly obvious as a direct response to the great films that came before him. I won't go into specifics because I'm tired and not anal, but Heat is all over the Dark Knight, all his other movies have been adaptations apart from his debut, and even Memento relies on its clever conceit more than its characters.
I believe Inception is a good film, but that it fails for the same reason cited above for Memento: it relies entirely on its conceit and expects a suspension of not only disbelief but the critical eye. I believe this is something Nolan does purposefully because he desperately doesn't want to be overshadowed by the canon of great films that came before him. And I believe this is something that fails in the face of Nolan's usage of Paprika's imagery and motifs.
To wit: Inception was a cleverly conceived heist movie that incorporated a lot of external influences, most importantly from Paprika. These incorporations, while the best parts of the movie for the uninformed, detract most from the solid plot of the movie in how clearly they are incorporated.