When people play it they're going to see it is, every bit, a "true" 3D game. People are crapping the pants and making a very big deal about nothing, going off of very little info, as per usual on internet forums.
For me I just want some good, original music tracks.
The "New" Super Mario Bros. series has crap for music, I know they think they are emulating the old Marios, but none of the new music is as good as the old Mario games. It just sounds like midi cheese.
I just don't want to feel like I'm playing a "paint by the numbers" Mario that Nintendo just sorta pooped out for the holidays. Level design is great and all, but I think the NSMB series takes it too far in terms of basically being just a "level design" showcase with barebones presentation in every other area and it's a little tiresome.
NSMBU is the first 2D Mario game in my life I won't play to completion, just because I got half way through and while it's a solid platformer ... I just realized I had no interest in continuning to play. The levels, while designed well just felt like formulaic and unfortunately I think the 3D Land/World series tries to emulate the NSMB series first and foremost.
I really like NSMBU, however NSMBWii is still unequivocally (at least in my mind) the best in the series. U has some nice touches to it, but Wii just had everything going for it: the first return of the Koopa Kids in about two decades, the first home console 2D Mario in that same span of time (Yoshi's Island was a Yoshi game), the Penguin Suit which holds a strong case for being the coolest Mario power up of all time next to the Tanooki, great level designs, far superior item inventory and Toad House system. You name it.
I agree that the music could be better. The "bwa bwa" style just doesn't really feel awesome. I'm with you there, though I don't think it's terrible.
However, Koji Kondo is back doing the music for 3D World, which is already good, and I have already heard talk of cool new versions of classic Mario tunes. So I think there's hope as far as you're concerned.