Room for alot of improvement.
The jumping was really dodgy, you kept moving too far after you'd let go of the arrow key which made simple things like jumping from platform to platform near impossible. The graphics were also badly resized in some places which leads to me say that either having the graphics bigger (as in the pixels double the size) or having the playing field smaller would've made things more pleasant on the eye. And is that a midi in the background music? If it is you totally change it to an mp3 of the actual music, or just download a remix and put that in. If you consider these things, it could be alot better! The action-script as a whole was pretty good, the jumping needs to be revised.