I've never seen Jaws because the commercials got to me when the movie first came out. It was the expression on the girl's face as the shark bites one piece after another off of her.
I've also never been interested in seeing Texas Chainsaw Massacre, mostly because they used to show commercials for it during the Warner Brothers cartoons I'd watch when I got home from school.
I'm having difficulty singling specific examples but there are a few things that I find too disturbing to tolerate. Violence against a helpless victim, especially with strong sexual overtones is rough on me. It's probably sexist but I find it harder to take when the victim is a woman and hardest with a child. It's funny because I'm fine with scenes of the pretty heroine or little child in danger, if they escape or are rescued. I can even handle the odd off camera death. It's when the camera lovingly lingers over the person's hopelessness and terror that it gets to me.
I thought Halloween was well done but it suffered for me because of this and I missed out on Jason and Freddy when they first came out because of this. Having since seen then on TV, they don't bother me so much. They're too cheesy to take that seriously. I don't have any plans to go see any of the recent 'tortureporn' movies though. (Big sissy that I am