A) Get some music and sounds (from NG Audio Portal, or Freesound.org)
B) Make sure that players die if they fall (surround the map with trigger that kills you and restarts the level?)
C) Make sure there is some way to know which objects are fake. Having to guess "which block ahead is fake" with no hints or logic behind it is no fun. It's tedious.
D) Physics (collisions) suck to the point where game is unplayable.
E) Don't respond to criticisms "it's a demo". We know it. People tell what is bad in the game so you'll know what to fix in the full version.