It's a really polished game. The color scheme was pretty. The sprites were also cool together with the music. I don't know whether that's just me, but I found it a bit irritating that the girl doesn't face the right side after you let go of D/RIGHT ARROW. But that's a minor thing.
You can go out of the map. You can also stay at either side of the map and that means that there aren't as many acid drops falling at you.
Maybe you could also make it so clouds come from both sides of the screen or change altitude? The more variations the better for this type of game.