I like that there's a No Fail mode that lets you finish a song and still record a score. I can finally play expert guitar and not worry about the solos kicking my ass (Smoke on the Water, damn you).
The ability to instantly switch instruments on the fly is a god send too, since before I'd have to log out and log on and blahblahblah. I'm just disappointed that they removed headset support, because that's how I would play the vocal and guitar sections.
I was all ready to upgrade to a RB2 guitar but it turns out all the stores around here got rid of them ages ago. I tried a GH guitar and I just can't play with it. The neck is way too thick and the clacky strum bar is just off putting.