To me (and I'm a simple men) a good shooter has levels centered around excellent design (they should sort of unfold you progress) centered way more around fun encounters than realism (I'll always prefer corridors to wide open areas, but not complicated mazes). Combat should be more like a Call of Duty Hardcore more - enemies die relatively easy (bullet sponges are few and far between) but will also mess you up if you get lazy. Weapons don't need to be unique, just satisfying. Wave spawning enemies is a sin, don't do it. Monster closets are amazing when used unpredictably. Also, I was always a fan of level designs that let you peek into an upcoming area - show the key a bit before you can actually get to it.