I'm a Rock Band freak, mainly I play the drums but I also like to sing (if you could call it that). RB3 adds a keyboard, which has me excited (I used to play piano and *cough* accordion).
The game is incredibly good so far: they addressed virtually all of my very few gripes from the previous game-- Your band members are there no matter what mode you're playing in; they added hi-hat functionality to pro drums; you can build setlists for quickplay and rate songs so that the ones you hate don't come up on shuffle; if you fail out of a difficult song halfway through a set you can still finish the set; keyboards are challenging but well set up.
The only disappointment is aesthetic. They went for a more realistic look, which is understandable considering the pro mode, but the body models are not as appealing-- badly proportioned, wonky anatomy. There is some degree of customization with the faces, but not enough to nail a look you are going for (it was actually easier to do in RB2 with fewer options).
One of my great pleasures in RB2 was watching my band members perform and interact onstage. The band members interact well, but since you can no longer assign attitudes to your characters they all look kind of the same. And since, inexplicably, you can no longer assign a stand-in band member to a specific instrument, they just play parts randomly-- you don't even get as much personality as 'this is the bass player, this is the drummer'. The drumming animation is a big step backwards. I actually learned a couple of things about drumming from watching my avatar play in RB2; now the drumming is stiff and lackluster. The crowd is the worst-- bad models, terrible animation, it should not have been allowed into the game in that state.
Of course, this is first-day "RAAR it doesn't look like I'm used to." But I also have a huge peeve about removing functionality from a game without clear reason, and I become attached to my characters, so I am really missing those aesthetic options.