Joystiq’s Coverage of “Rock Band 3” 

A few of the major new features had leaked previously, like vocal harmonies (a holdover from Beatles Rock Band), a new instrument (a keytar) and a "Pro" mode that makes playing Rock Band more like actual music. But wait, there's more! Ends up RB3 was a ground-up rewrite of the game, with a focus on making the game easier to set up and play:

Across the bottom of the menu screen is something Harmonix calls "the overshell" — for every member of your band, there's a little popup menu where players can set, using their own instruments, their login name, their instrument track of choice, their own difficulty setting, lefty mode, and any other options. That's right — no more will your Rock Band parties feature one awkward person yelling out, "Guys! Don't touch anything! Just let me set it up!" Everything works seamlessly — as one person navigates to the right play mode, everyone else sets their own tracks up and chooses their own options. Additionally, the overshell allows drop-in, drop-out anywhere, anytime during the game — as Harmonix told us, "this isn't a separate team mode or a party play style," it's everywhere in the game.

Naturally there is also a new setlist (including "Bohemian Rhapsody"), and as with Rock Band 2 you'll be able to import all your songs from the previous game for a small fee.