This is the rhythm game I never knew I wanted!
The one-button mechanics for either one line or multiple lines is so simple that it works well. It also really helps that you set up each beat segment so the player learns how to handle it before introducing the next. This is especially well illustrated in levels with more than two lines.
The songs are fantastic, with one exception: Matchbox Twenty. I know you were just experimenting with how well this concept works with pop songs, and I'd have to say that it doesn't work. I'd recommend concentrating on the kind of lyric-less songs that occupy the rest of the game.
Level two is so sweet <3, the boss is surprisingly epic - who knew that kind of glitchiness could work so well in a rhythm game?
(Another recommendation: If getting a perfect on the boss level adds to your NHS Approval Rate, then there should be some way to track your rank of the boss level on the menu screen. It's not a big deal right now, but if you add more bosses it will be.)
The claps of the seventh beats, when they're all pressed correctly across different lines, really make a great rhythm together! And that adds to the game feel a lot.
Overall the presentation is stellar. I've S-ranked everything except the bonuses, though it was a challenge! I hope you continue to develop this game, either in flash or even trying to move it to Steam or something of the kind.
Now if you'll excuse me, I want to go download those cool songs!