This is rather unexpected. Considering this would be a spoiler to many, we decided to put this one after the break. You'll see the last level in action, and you'll find out what really-special-feature unlocks at the end of the game. For the impatient ones, skip to 5:20. (It's best watched with full annotations, here.)
[Thanks, jimb! Amazing video captured by Tntnnbltn]
So it appears you unlock custom soundtracks. We haven't gotten that far yet, but we can't wait to unlock that feature.