well I am not big on FPS( I like Thief 3/KOTOR/Mass Effect games) and that's mainly because they are so unrealistic the only FPS game I played longer than a few hours was Killzone 2.
There is never a story which really connects you the player to your teammates or the main character itself.
All bullets fired seem to be aimed solely to the player, I once played the crysis demo and at a certain point I reached an enemy base , I made myself invisible and shot one from a rather large distance, the moment I fired it seemed that all crosshairs of all conceveable surrounding enemy players where aimed at me and I was dead in like 10 seconds, I watched videoreviews of the above mentioned games and they both seem 13 in a dozen games.
I know they are just games but I am really annoyed by the fact that when I see a live demo of someone playing they seem to be running at insane speeds through corridors, running at the enemy like rabid dogs, which to me already breaks the game.
They should make a game where one hit could mean your death, teammates or yourself getting injured and need to be dragged along or left behind in order to succesfully complete the mission...no disney endings, but the reall deal.
I also hate the fact that those games make killing easy, always have clear good-bad sides, I would like to see that in the beginning of the game when you just enter the frontline camp that one of "ours" is bringing in a POW who begs for his life, crawling towards you talking in a foreign language and then he shows you a picture of his family ( the standard picture: lovely wife laughing kid etc) the camera focusses on the picture and then you hear a bang.( blood spatter on camera)
You look up the guy is dead and one of your "own" laughs and says : " I always hate it when they beg..."
Between every mission there should be a rest period with RPG elements where you connect more and more with your teammates so that in the end you really care when they die and start hating the enemy, killing POW's like nothing ignoring their begging, I would also adjust the colorpallette of the game desaturating more and more up until the climax, making it a very grim game where you might be the only survivor and all your friends die.