Pretty fun game.
Hard mode was a nice challenge. The arrows should respond to a late arrow press so one could time the key press on the beat rather then having to hold down the key moments before it hits the bottom of the screen.
Also, It would be nice for the arrows of specific directions to always appear in the same location horizontally, like in DDR. It just makes it easier to read.
More songs wouldn't hurt.
Finally, you could have both the note things flying down and the arrows coming simultaneously, to make you think more. Maybe a "Very hard" mode?