Gasp!
Funny thing :D! I was about to make a game exactly like this (but with 4 buttons) and submit it to the Rock Out competition xD! (I didn't because Coolio asked to collab on the Music Mania thing and I figured, I already submitted in too many.)
Several things...
1) The easiest song is so hard. Using the mouse to "strum" is a great idea except the problem is, it's not a common idea. In my opinion, you should've made an easier song to start out with. I got up to 9% and failed xD!
2) When you go to the instructions in the menu and go back to the menu, the song begins to play twice. You probably would've noticed this if you tried to find for glitches after you were done.
3) More responses, perhaps. I would've appreciated it more if it had some "Hit!" or some "Miss..." signal after you hit or miss a note. I can't see the difference between either of them (they both just reach the line and shrink). Either that or I've been missing the whole time...
Well, that's all I can think of right now. And like the person under me, this should not be blammed.