Terrible. Needs more internal work
I know it's just a "demo", but even if it weren't, this game isn't ery entertaining or good at all.
For starters tthe engine you used is terrible. I have to wait a few seconds for my caharacter to move, and when I jump, I have to press the arrow keys again to move in the direction I want. You should make it smoother and more stable because your hittests were all crappy. Try creating one yourself and not copying it from some crappy enginee someone posted on the BBS.
The graphics obviouslyneed major work. Try adding exotic backgrounds, and shit. The character looked fine, though :)
Finally, I recommend adding a story, because a platform plainly sucks without a goal or story in it.