So here's the games Story from Nintendo's official NSMBU website:

THE STORY

"It was just another lovely dinner at the castle with Mario™, Luigi™, Toad™, and Princess Peach™... Until suddenly the ferocious Koopa fleet invades Mushroom kingdom airspace, and launches a full-on assault! With Bowser™ at the helm and his Koopalings each in their own custom airships, the castle is under siege with the Princess trapped inside—and the heroes have been cast out to the furthest reaches of the Mushroom Kingdom. Can they make it back in time to save the day"?

Notice Mario and Luigi have to get back to the Castle, instead of Princess Peach being kidnapped?

Though its not a drastic change in Mario formula, at least Peach is waiting for the Bros!

Interesting, but basically the same.

I have a feeling this game is going to be REALLY long, because not only are the Bros. going on an adventure to get back to the castle, but I guess that by the time they get there, Peach will have been taken away to Bowser's Castle.

This has been done before...

Anything new has been desperately needed and is welcomed. Not enough that I want to buy it for even half the price it will sell as, but hey...IMPROVEMENT!

yeah since it was basically said that there will only be one NSMB per system, I've lost most reason to hate on NSMB. If it's not gonna be a constant stream of these games like an annual release CoD, I can't really get that annoyed with the games.

Yeah, it is getting away from the norm, which is a good thing. Although this idea has kind of been done to death in the spinoffs. Remember Mario and Luigi Partners in Time and Bowser's Inside Story? Both had hijacked version of Peach's Castle as the final 'dungeon' with Peach held prisoner inside.

While it's still a plot revolved around Peach getting kidnapped, it's at least something a little different, which I welcome. Actually, Peach is taken hostage, not kidnapped.

While the idea of Bowser occupying Peach's Castle has been done in the past with Super Mario 64 and Paper Mario, it hasn't been done with a 2D Mario game, so there's the difference. The only event that was similar is Wario's occupation of Mario's castle in Super Mario Land 2, and that was with different characters.