Although i havent been using Hammer for a while, i have been playing HL2 DM for about a year, and of all the custom maps ive seen, the most fun are the ones that invent a new way of playing the game.
For instance, i was playing on a map called "Buggyville_1b" and the whole map is a criss-cross of hallways and the only weapons you find are the rocket launcher, combine rifle, and the combine energy balls. The player has to adapt to the new way of fighting, and it makes the level a lot of fun.
So i guess what im saying is: Try new things and go out of the ordinary, and you will probably come up with a really fun level