I have to say, I'm deeply impressed by the variety that Nintendo crammed into this game's levels, especially in light of how they never change the game mechanics mid-stream like they do in Galaxy.
Like SMB3, each level has its own unique feature/gimmick, but unlike SMB 3 there aren't many levels you can blaze through in under fifteen seconds. Like SMW, most levels are packed with secrets and alternate paths (despite their apparently linear nature!), but unlike World this game manages to make each level individually memorable. And again, they do all of this without once changing the game mechanics: at no point does this platformer morph into a rolling game, or a puzzler.
Hats off Nintendo: you plum done good.