Loved this game :)
I had some problems, on the second time I played I couldn't jump. I had to refresh the page. All the times I played it happened.
The animation, if you jump again when you are jumping, it restarts a bit, I guess. It happens something that seems it wasn't supposed.
The music's fantastic, it completes the enviroment of the game. Way cool. You should gives us the mp3 to listen :)
I did enjoy too those pixeled clouds, nice touch.
The only thing I think you could get better is the jump system. For example, when you stop pressing the Space Bar the Character could stop jumping as well.
The jump could last only the time the player presses the space bar. This way it would be possible to control the 'air time' (as long as you set a maximum jump arch) and it would give opportunity to do small jumps, which would come in handy.