What ruined FPS' is the move away from open, nonlinear levels with limited "objectives" to railroad "cinematic" shooters.
Games like CoD are all about the cinematic "WOW" factor. There really isn't a level, just a few corridors that lead the player to each "WOW" point in which the game stops and waits for the player to complete a retarded task whether that be kill all enemies, kill a specific enemy, destroy a specific target, or collect a specific item.
And thus, in CoD4 you can waste hundreds of Iraqis while the game waits for you to destroy a specific tank. If you don't, then you never advance and you never run out of enemies (or ammo)
Obviously this gets boring and it also doesn't leave room for replay. So while we can play through Doom over and over again, there's little point in playing CoD4 again unless it's really that fun to "Destroy the specific wall so the troops can get through it!!!!" again.