Each level, whether user-submitted or part of the game, gets its own feel or ‘ambience’. An ambience is made up of things like the music, lighting, and how the non-playable area outside of the level is rendered.
Up until now you've only seen the outer-space ambience on this blog, but I've just finished an ‘underground’ one, and more are planned.
The ambience system helps to keep each level unique, but, if people are going to be submitting lots of levels to the Level Pit, just having three or four different ambiences won't cut it. That's why ambiences are designed to be infinitely variable — for instance, the Underground ambience generates its rocky surfaces procedurally, and the texture maps are different every time.
Ambiences are selected and tweaked automatically by the game engine based on things it notices about your level. For instance, if the level isn't entirely surrounded by walls, it won't be put underground.
How about a claustrophobic ambience for maze-like levels? An angry one for enemy-heavy gauntlets? Rare and secret ambiences triggered by some obscure wall formation in a level?
More to come.