It's a solid and respectable Mario game and I like the challenge mode, but even within the NSMB series I would place the sadly maligned NSMB2 above it on the basis of level deign and natural feeling control (better sense of inertia, which gives it a stronger sense of physicality imo). My main gripe with NSMBU is that it all feels a bit too rote and smoothed out, polished but not inspired. I didn't ever feel engrossed or compelled to marathon it, which is something SMB3 and SMW inspire even after decades filled with countless replays.
It lacked an addiction factor that for me separates "merely" well made games from a stone cold classic. I know subjective terms like "magic" or "creative spark" are frowned upon in these discussions, but I do think those things are felt when a game surprises and delights you in new and exciting ways. NSMBU needed to be less predictable and more idiosyncratic and forward thinking to stand out as its own thing rather than yet another NSMB game. I know not everyone puts equal emphasis on that sort of originality, and I wouldn't even claim it's the end-all be-all, but I think even with the passage of time you can pick up on when developers themselves were really discovering something new when making a game. There's a vitality to games with emergent elements, while NSMBU feels more like an admirable refinement of a well worn formula- but still just a refinement.
That said, I was happy that it put more effort into the aesthetic than previous NSMB games, with lusher more nuanced backgrounds, and a more fleshed out and interconnected map. If they were going to lean on a past success, SMW was a good choice (just like I thought SMB3 was a fine choice in the case of NSMB2). It's got a nice amount of content, both in the main game with the side stuff, and the multiplayer is something that separates it from old school games and ensures it will always remain on my shelf ready to play. It's also just nice to have a modern Mario platformer in HD with all the bells and whistles in spite of some of the sterility of the samey NSMB approach to level themes. Although I wish they had gone out on more of a creative limb they did a good job within the confines of the standard NSMB approach.
So, I'm not quite on board with the OP, but I appreciate the scope of the game and its place within the series. It ties together a lot of the existing strengths of previous NSMB games, and rests comfortably with a small handful of other modern retail platformers that scratch an itch no other genre can. And at the end of the day, that alone is pretty awesome.
