there are other factors wich make or break a fps i think , the caracter being one of them , the other thing would be that the game needs to slowly progress from "someone trying to survive" towards "a hardened kickass force to be reckoned with" this is apart of what makes the story line i guess .

look at most of these games they mostly started with you being very vulnrable and under equip't for the task at hand and then as you progress through frightening battles and challenges you are rewarded with little perks of better weapons and such untill later you become what the "enemy" fear , the tables turn slowly through progression towards the end of the game , and when your done playing yoy must feel as though you made it through hell and overcame everything , trying to get the player to actually feel some kind of emotion is key .

then theres other things like ambiance take for example the old unreal games with its mysterious exploring music and all of that silence and little sounds adding to the feeling of the game , unreal was a very different kind of experience for me because of this , it wasnt just shoot them up but it had moments where the player felt like being drawn deep into the games world ..

some of the most fun aspects is the physics of bullets or projectiles , getting used to how the weapons behave and being able to feel where the next shot is going to impact makes things so much more fun , i think medal of honour or it was call of duty , one of them had great weapon ballistics (not realistic but fun) a good example were the last wolfenstein i played .

ai does not have to be too smart they have to atleast make you feel you are engaged in a battle atleast and having them positioned at the correct strategical positions along with just enough of them in numbers plus that one surprise enemy waiting for you could help ai seem more fun .

i think when you develope such a game you start with pen and paper , lay down the history of how the player came to be in the wrong place at the wrong time , and create the world and its history and story and caracters all before actually stepping on to concept art , code when you think you have everything planned out ..