NSMB2 introduces less to the formula than any of the NSMB titles, but the actual level design really surprised me. By the end I felt it was a tighter, more inventive (within the confines of the established assets and mechanics) than NSMBW and of course NSMB. The lack of innovation and its heavy asset re-use is about the only serious flaw I have with the game design (various coin mode related complaints, but they're different). I was very surprised by how solid of a platformer it is.
So much so that I'm wary of NSMBU matching it. Even though I know NSMBU is bringing more to the table, which is fantastic, that's not enough. The level design needs to keep up to scratch. Fingers crossed it does.