so, what is good level design?

1. short walks. let the player have action, adventure, or whatever because he is playing a _game_, not a landscape simulator.
2. few repetiton, it's no good sign if you have already seen that corridor once. and there's another one that looks exactly the same. bah.
3. revisit places from different angles/in different states. e.g., you crawl under a room in a vent shaft, see a lot of guards, then enter the room and know where they are.
4. connected rooms. you see a door that's closed, have to make a detour, and can open the door from the other side, so the level is "connected" and not just a long corridor with some rooms or a plane with buildings.
5. multiple levels (height). don't forget that you are in a 3 dimensional space. i dont only want to go left and right, but also jump, use elevators, ladders, crawlspaces, sewers etc.
6. nice, yet unseen architecture.
7. central map objects like a reactor core, a prison, a bridge, or whatever.

terrains certainly fail in 1, 3, 4, 5.