This is an incredible game.
The one thing I find jarring is the song pausing until you hit the proper key. I'd take some elements from Guitar Hero and Rockband here. So scoring improves the more accurate you are with timing, but if you're way off or hit the wrong key you get no points or even negative points. The important element is that the song doesn't stop regardless of whether you miss a note or not.
I'd also make the guide above the keyboard more informative. There could be a few bands that indicate levels of accuracy with a distinct band for perfect notes. I'd also make a more obvious visual effect to show when you've hit a perfect note, gotten close or have missed completely.
This is not to say I don't think the song stopping on a missed note is bad. I just think as a gameplay it isn't as effective. I do think it would be a nice feature for a practice mode, so that you can actually learn the song without the pressure of trying not to miss notes.
It would be fun if you could see the ice pianist in the background playing the current song, add some additional visual interest, although I realize that that's getting a bit crazy.
Otherwise, like I said, I think it's a great game.