Judging this as a demo, which it primarily is: this is incredibly awesome!
As a game: there needs to be some way, on the more sparse levels, for falling down to not be the end. The obvious answer is to give the marble some kind of upward mobility, but I'm thinking it might make sense to give the player the ability to morph the fractal to try to make the terrain more favorable; that'd fit the theme and show off the fractal collision tech.
Given that most people are going to be here to see the fractals and not really for a challenge, I strongly suggest the level select have all levels open from the start. As it is the level select sucks; you can't even go back to the level you left off from.