Simple and entertaining. As others have said, the difficulty curve makes the game unplayable way too fast.
It would be nice if stomping on an enemy gave more of a bounce than it does. That way, you wouldn't be completely screwed when you were facing a huge line of enemies. You could just bounce from one to the next, and that would extend the playability of the game. Also, I wouldn't make the ladders hurt you when coming up. I feel like that's a pointless annoyance.