A Good Map
1. Good shading. Like setting a tree close to a building and then putting another (wall sprite) tree on the building + making it completly black so it looks like the shadow of the tree. And in a sector where there is a fire or torch. Make it flicker. And then there's the simple stuff, like wall shadows, etc.

2. Surprises that test the player's reflexes. There's nothing like a good show of moving sectors when, all of a sudden, the show turns into a death trap.

3. Players love to blow up things! Make all glass breakable. Windows, vases, glasses, and maybe even statues. And a little something that I learned myself. Make a table or wall out of sprites. Then make it blow up into burning chunks of wood.

4. It's kinda old, but you need something right behind a door. The best door to put a cultist behind is a door that opens from the middle. In case you don't know how: You simply make it like you would a regular up&down door. Just raise the floor and lower the ceiling. When they meet in the middle hit ALT-F3 for CLOSED POSTITION. Then lower the floor and raise the ceiling. Then press ALT-F4 for the OPENED POSITION.

5. And then there's the mood of the level. Make it dark and have some wind and distant crashing or yelling to make the player wonder what's out there.
EmAiL mE