Escort's probably the most annoying if the person can't defend themselves. Okay, he/she is a civilian, but I'm pretty sure if they have at least one hand with fingers attached, they can fire a gun. They might not be accurate, but given the circumstances , don't think anyone would really mind.
Another big issue is stupid AI, it's not so bad now (probably because escort missions have died down somewhat) but I used to hate it when I called a person to go a place or follow up behind me, they'd run into an obstacle and were stuck until i changed the direction they were walking towards by running all the way back, just to stand beside them for them to turn and walk around the object... pathetic
Defend the place is not so bad, except when they give you no mounted weapons or auto defenses (including mines) (i.e End of Call of Duty 2 on the hardest difficulty was such a pain). I didn't mind the mission in CoD4 though, at least you had some mounted weapons, c4, claymores and air strikes to provide support. I actually kind of wish they had that sort of mode in CoD4 (closest would be domination or headquarters, but I'd prefer the teams either doing solely defending or solely attacking, not just trying to balance both).