I could see an ultrarealistic multiplayer FPS being fun, but I just can't see ultra realism working as well in a Single Player game, since, whenever you die in a Single Player game, it's a road block, not a fun factor (except in Planescape Torment, but that's not realistic now is it), hence frequent death is not desirable.
For a single player game I would say that a focus on realistic combat tactics and a realistic injury system (limping, blinding, disabled arms, bleedouts) and a simplistic healing system (you just need to get to a medic/health kit etc to fix yourself up) would be a workable combination, since needing to find 10 different healing items to heal each type of injury is tedious, not fun. As well being able to fire through walls, having ricochets, long reload times etc. are all a necessity. As well I would like to see climbing be an option for once, and you really shouldn't be able to "jump" since in RL nobody jumps, you can't jump very high, you step over, leap up, or hop over, rarely do you jump, hence a "jump" button is irrelevant.