I really enjoyed this game, I loved the concept, the levels and also all the music and effects and sounds are really engaging!
One thing I believe is to be done better is, like others said, the problems about the acceleration of the character and the way he moves and jumps. Too many times I knew how to solve a level and I couldn't just because I pressed a key a fraction of second too long or short.
Also I believe the r key should restart the game faster. Maybe the animation doesn't need to be much faster but at least at the end of it I'd like to see the character immediately ready to move (now it takes a short little while, short but painful if it's the seventh time you're playing the level)
Thanks for this great game!