I don't feel dissapointed. It was a sequel to NSMB on the DS, and pretty much did exactly that. With improvements in nearly every field.
That said, I do have to say that the game is 'harder' as the developers stated. But I feel it is somewhat harder for a different reason than usual platformers or even certain levels in past Mario games. It feels more like most of the levels in this game are built around trying to hide the star coins and make Mario avoid things (enemy projectiles/falling objects/lava/you name it). The game seems to have a one track mind when it comes to level design and, while still fun, feels somewhat cheap. Especially on certain levels because you find yourself getting hit by enemy projectiles when you just see them coming. Yes, that is one way to increase the difficulty of a game...but its a reather cheap way.
All in all however, the game overall is still great and doesn't suffer too bad from my above scenario. But after playing nearly all of it (still have to finish the extra world and find a few more star coins), I don't think I can say its one of the best 2D Mario games. And even the level design in NSMB DS seemed to be more varied than this game.