Good DM levels have multiple entry and exit points to each room. Dead ends should be avoided, unless they're carefully used with a highly desirable goodie. Make sure the bots work well in your map. I find that starting a match as a spectator with one bot in a map and watching him repeatedly will often reveal problematic areas that can often be fixed just by moving a pathnode around. Try to have a spot for nearly every weapon.
Every weapon has a situation for it as well as a player who loves it.
It's very frustrating to not have any visual indication of such an impressive defensive system. That's why the shieldbelt works so well, you can see that they have this great defense as you try to strip it away from them. I like maps that let them stay up two seconds or less, and then they come back down. Try to avoid lifts where your head hits something and the lift returns to its original position. Never make a lift crush a person.