Guitar Hero II
I played the demo from OPM this morning. Not too much is different from the first. New thing to watch out for, though: three-fret chords. One of the demo songs on expert alternates between two different ones, and I almost lost because of them being completely unexpected.
New feature which I think is good: whenever you strum at the wrong time, the board-thing that the prompts scroll down on shakes a little to warn you. Very helpful to know when you got those licks wrong.
I'm really looking forward to this game, as I am very good at the first one.