All the characters are now one year older, so some of them are now significantly taller and/or have different hairstyles:
Makoto now have a rather feminine hairstyle, Miki, Iori, Azusa and Ritsuko also changed their hairstyles and Ami & Mami are now pretty tall and not a mirror image of each other anymore, so they act as two independent idols. The rest seems pretty much the same.
The gameplay is now focused in manage a 3 girls unit, with special emphasis on the trio leader. The details are pretty vague for now, but it seems that 5 girls units are also possible:
And finally, perhaps the most shocking new is that Hase Yurina aka Yurishii (formerly Ochiai Yurika), the voice actress of Yukiho, left the project and the new seiyuu will be a fairly unknown Asakura Azumi.
Yurishii left by her own, and considering all the unlucky events that happened before, is understandable.
As said, for now is an Xbox 360 exclusive, but considering that another Bandai-Namco game, Tales of Vesperia, was also an Xbox 360 exclusive and was ported to PS3 a year later with additional contents, there's still hope for the non-owners of Xbox.