For some reason when I played it the multiplayer was fucked. Sometimes players entered a downed state after taking damage, at which point they could shoot and kill you with their pistol, but even if you walked right up to them and emptied an assault rifle magazine into their head they'd be invincible.
I also dislike perks, loadouts and multiplayer levelling. MP3 multiplayer was shit, even if the single player has a special place in my heart.