I think they should use actual...you know...notes. I want to see something like (As apposed to different colored notes coming up) A, B, C, D, E, F, G, 2A, 2B, 2C, 2D etc. (The numbers in front of the note indicate the octave.) This way, it can still be used by non-musicians (They'll figure it out) And actual musicians can transpose songs or write their own.
It would also be nice to string note blocks together so that when one finishes, the next starts, that way we don't have to make overly elaborate redstone circuits, and then we'll have to trigger each block separately! >_<
It would be great to connect note blocks with redstone together, and when one is triggered it plays, then it sends a message down the outgoing redstone wire kind of like a button instead of having them touch directly to do the transfer. This could be really cool in making a song play with just one button without an overcomplicated work around.