Too much guesswork
The game had promise, but there was too much guesswork involved and the physics was too incorrect. Your ball should not lose horizontal momentum when striking the top or bottom walls, and the angle at which you bounce off of a normal face should have something to do with the angle and speed with which you hit it.
I liked level 19 once I figured it out though. Deceptive. For lots of the levels, though, you could fire the ball off randomly and wait long enough and the level would play itself. That's no good.