Assuming that P != NP, I believe it has been shown that there are problems which are not in P and not NP-Complete. Graph Isomorphism is conjectured to be such a problem.
Is there any evidence of more such 'layers' in NP? i.e. A hierachy of more than three classes starting at P and culminating in NP, such that each is a proper superset of the other?
Is it possible that the hierarchy is infinite?