Darkness and ominous ambient sounds are all that's needed to get my heart-rate going. My imagination does the rest. Sudden movements at the edges of the screen really get to me as well. I couldn't do the Cradle in Thief 3 though and the haunted mansion in Vampire: The Masquerade was borderline too, even on a second and third play-through. :/ I was fine with everything the Stalker games throw at me though.
I need to man up and play Outlast or A Machine for Pigs, just to see if I can acclimate myself to it. I should be OK with some break in-between session.