Rock Band excels at fulfilling the “hit button and get a pleasurable outcome” center of my brain. There is altogether satisfying with getting together with your friends and pretending to play a song that you know.
When one of you fails, you all fail. When you all succeed, it is a moment resplendent and glorious. There is laughter and satisfaction and to a certain extent, accomplishment.
The mood altering combination of performing with friends, listening to music, flashing lights, coupled with the threat of failing together comes together to create an experience that resembles a piece of performance art, rather than the mainstream definition of a traditional video game.
Also, it is wicked awesome to rock out.