The difficulty curve . . .
is pretty steep once you hit world 2. Nothing wrong with difficulty, but after you've gone to all that work making moving very quickly to the right fun (as all hell, don't get me wrong) you go and put us in spiky vertical labyrinths every other level. Just breaking up the later levels with the occasional "hold down right and jump" section is all it would take...
Other than my whining, well-designed game here.