The one thing I absolutely loved about Portal were the "behind-the-scenes" maps. Finding clever ways to advance like redirecting rockets through portals to break pipes to acquire cubes, finding abandoned rooms with easter-egg like material, rooms full of turrets, moving piston-like pipes, and having to multi-fling to make it to a certain point in the level.
Albeit the challenge courses are fun. Figuring out the puzzles and unlocking least times, portals, and steps. I'd love to see more of the "back alley" find your way out and survive environments. Most custom maps make use of the standard examination rooms and turrets but none have yet used the rocket launchers or the guts of the complex. So while all of you talented mappers are hard at work, don't forget about the other side of Aperature. There's more to it than just creating neat puzzle rooms!
EDIT: I forgot to mention FIRE PITS! Lots and lots of FIRE PITS with moving platforms teeming with obstacles that you have to try to portal around while maintaining a stance on the platform. Maybe even throw in some of the charred zombies and lay them about the pits to show failed test subjects!
