I loved the design of the various characters, and I liked the gimmick mechanic you added to each one. (My personal favorite wound up being the Skull Boys: they were adorable, and I loved how their animations really brought them to life!) It was a generally simple game, though, so I found myself not needing to use the character's special abilities that often. Like some other people mentioned, I think some more animation and different music could have added a lot. Since it was a band/music based game, I think adding more background tracks in particular could have been interesting.
It was a fun game over all, and I found myself very quickly racking up level after level!