Escort levels.
I know what you mean. They are some of my most hated things in video games. I think the video gaming industry should have a moratorium on escort missions/quest or things where you need to keep a certain person alive until they can do them well. I often find either the AI of the person your trying to protect or your other team mates to be completely stupid where they try to get themselves killed, or that the computer AI enemies (and especially human enemies if your playing against another player) will do kamikaze charges against who your trying to protect even if it wouldn't happen in a real life scenario like that because the risk to them would be too great.