Almost anything. I'm very easily startled, and actually just knowing there's a chance I'll be startled will most of the time get me to quit the game.
This is within some (un)sensible boundaries, i.e. not stuff like Call of Duty's campaign or some such (i.e. perfectly natural nazis hitting you in the back with guns or whatever), though nazi zombies... yes.
I do usually get used to these things. First when I tried to play the L4D demo I freaked out.
It's usually the sound, i.e. if I don't have the sound on, or don't have my headphones on I'm less likely to be startled, but it's a very annoying debilitation.
So far these are the games I've tried and not been able to play due to being startled/knowing I'll be startled: any HL game (I've 'played' HL 1, HL2, HL2:EP1 and HL2:EP2), Far Cry, Thief 1 (zombies, I freaked out, but then again I was about 12), Thief 2 when I reached a cage full of spiders (yes, I hate spiders, legs vs. body proportion = creepy), Fear.
Most games I know will startle me I'll usually not even bother trying to play, so the list isn't long, but it could be.
Not scared of the dark, graveyards or stuff like that (less so than my friends), but I'm unable to play things that startle me/I know will startle me (a lot, that is).
I'm 16. I think I should really do something about this thingy, stopping me from playing games that are generally considered good (though to be honest I've never liked HL, and it's not because I can hardly play it).