Baby-sitting, by a long shot. This is alleviated quite a bit if the target is actually durable and/or fights back proficiently, but helpless innocents that game over your ass if harmed are tremendously annoying, and more often than not the game does nothing to make me care about these people anyway. Forgiveness to Half-Life, where the escort portions usually only made you drag some scientist though about 2 rooms which you have the chance to clear out first, and most of the escorts were optional anyway. Likewise, I don't mind it much when the person you're escorting is ignored by the AI opponents, is just plain invincible, or the penalty for failing isn't that high (all City of Heroes escorts fall into one of these three categories). For some reason, I don't hate the baby-sitting part in God of War either; maybe that is because the game actually gave motivations to the character and the fight is pretty easy on normal difficulty.
Defense is fair enough, but the genre is important. In FPS games, its very difficult to protect multiple areas by yourself, so expecting you to guard 3 objectives with a significant distance between them by yourself with constant waves of enemies is a big dick move. In an RTS game, this would be a fair mission because you can have troops at all 3 points. In an FPS, there needs to be reasonable time to get between objectives, useful teammates to guard the other ones, or some other aid (such as turrets in HL2).
Grinding: I'll tolerate this in an MMO. It'd be impossible to make them long enough without. But in a non-MMO? single player games with grind can fuck right off.
Dark room with flashlight: In general, I don't mind this; Left 4 Dead makes great use of it. However, some games overdo it. Doom 3 would be a prime example of when dark rooms crosses the line from scary to absolutely fucking annoying.
Timed Missions are fair for most games, but they need to be placed properly and given proper sense of urgency, like if you just set off some C4 on a nuclear power plant's control system (Fallout 2). Timed missions mix very badly with open world areas, especially where you should be encouraged to explore, but for linear missions they can work well. In general though, the game should either autosave right before the timer starts or give a long enough timer that anyone competent can make it on the first go (exceptions for higher difficulty settings, obviously).
So, I guess any of these have a proper place in gameplay, but they have to be used responsibly. Any one of these can seriously fuck up the fun if used incorrectly.