Awesome proof of concept. With a little refining of the controls and some more levels this would make an excellent game. I like the drastic way momentum is lost if you make a mistake, and the time it takes to get it back, it makes mistakes feel meaningful - however, sometimes the mistake isn't the player's fault. In my opinion the jump is ever-so-slightly too low, I'm talking a few pixels' worth of height, just enough so that the player can get over ledges from a standstill. That, and for some reason the jump is sometimes VERY high. Very fun! Well done :)