Another competitive FPS person chiming in!
Most of the basics have already been covered, but a HUGE pet peve of mine is when map makers scatter crap all over the ground. NOTHING and I mean nothing will make a competitive player rage at your map faster than if they got shot because an immovable tin can or the curve of a sidewalk turned their jump out of the way of fire into them jumping in place and getting killed.
Getting stuck on a piece of debris is rage inducing. If you're going to put stuff on the ground unless it's as big as a crate or barrel or bigger, make it non-solid. This includes road cones, small cardboard boxes...
It's quite simple really. I want to be a professional level designer. I want this to be my job.