This is obviously a small experiment, but a handful of important points:
(1) give visual feedback when an arrow key is pressed and on the timing of the press when hitting a note
(2) the arrows shouldn't be partly transparent, that simply makes it harder to parse; also, darkening the "track" where the arrows come down would help, too
(3) gamepad support! These games are so much better when you have a kind of mirrored control (where the dpad and XYAB do the same thing), letting you use both hands in complex patterns (obviously not important in this game, but still)
Other than that, perhaps you should try the game Sequence (Before the Echo), which is a much fuller version of this; one of my favourite rhythm games.