Having finished the story (but not gotten all star coins yet), and thought about it for a few days, my feeling at the moment is this:
1. Yoshi's Island is still their best platformer, and in hindsight / modern framework YI is practically "Galaxy in 2D".
2. Having fiddled with SMB3 and World recently, I probably like NSMBU better than them, overall. It's definitely better than World, but arguable that a lot of the classic courses in SMB3 are top-tier 2D Mario. But U obviously can do a lot more with creative stage elements and bigger, splashier ideas. It is a tradeoff.
3. The refreshed world themes are enough for me; most of them feel different enough from NSMB Wii. Sparkling Waters is the one that feels the least improved thematically.
4. World 8 / Peach's Castle was a disappointment. It's surprisingly short, as if to balance how big some of the other worlds are. They could have done a lot more with a "Mushroom Kingdom under siege" theme.
5. Having said that, the payoff of the final boss fight does make up for it. A spectacular boss IMO, and yes, visually the most impressive sequence in the whole game.
Overall, I'm really happy with it, do think it's the best NSMB game, feel it's better than Mario World in every way except music. As far as music goes, I don't hate bah-bah. And in fact, I think the new Acorn Fields theme and the atheletic version of it are really good. Best music in the NSMB series. But I do wish they'd have made new music for worlds like Sparkling Waters, regardless of whether it had bah-bah or not. And a new castle theme.
As a launch game, it's more than big and meaty enough with much better content than typically seen in launch games (that aren't ports or upgrades from a previously existing game). So overall, I would say mission accomplished for returning to the standard of launching a Nintendo system with a quality Mario game.