-You need..
to increase the frame rate to 30 frames per second so the movement looks much more smooth.
-Fiddle around with the actionscript to better the chance of not jumping out of the screen.
-I found that I could jump while in the air, wtf?
-And finally you need better buttons because I had a hard time clicking the first "play" button, and chances are, the rest of your audience will too.
Please revise your flash so that it doesn't suck so much hairy cock when you submit the full version.
P.S- Don't reply with anything saying it was only a demo, that's no excuse for laziness and over ambition.