Multiplayer in the original DJ Hero was a plain vanilla affair that stripped some of the coolest elements from the single-player game for simplicity's sake. DJ Hero 2 throws simplicity out the window.
Forget adding a singer. That's certainly novel and the opportunity to mix up the music they are singing to screw them up sounds entertaining enough, but I don't really see it as a selling point.
No, the selling point for me is the ability to rewind in multiplayer. This is what they left out of DJ Hero that killed the multiplayer for me. Once you gain enough power you can spin your deck backwards, rewinding the track to score more points. In DJ Hero, this was left out of multiplayer. In DJ Hero 2 the feature's been left in, making your opponent stand there looking stupid while you rack up a higher score.
It's that sort of theatrical spin (pardon the pun) that could make DJ Hero 2 the part game DJ Hero was meant to be.