Great concept for a game, and I think it matches the theme perfectly! However I do agree with everyone else on the graphics and audio side. Also, the movement threw me off a little bit, wasn't expecting to keep moving for a half second or so after letting go of a key, but I got used to it :)
I think introducing different "worlds" that have a set of levels and music matching a theme (e.g. Space, Temple, Mountains, etc) would reduce monotony and make things more interesting. This is only a suggestion though, you do you!
Great job!