(edit:whoop this was sposed to be a double post, thanks wyatt)
gore/bodily fluids are never that scary for me. i grew up with a lot of sick people and saw enough of it to desensitize me. also i've owned a lot of animals and they're constantly spewing some kind of muck. i've also babysat a lot of kids ... when something or someone hurts themselves around you, it's very useful to be able to stay calm and know how to patch them up. i can watch zombie movies or slasher films without wincing. when i worked for a lawfirm a while back, and there were auto accident injury/death photos that needed duplicating, people would hand them off to me, because I was unfazed by them. i'd see some really horrible ones, too--corpses that were sheared in half, or ground up under tires, or spread out like paste in the gutter. gross but not scary.
the concept the lurking unknown is what gets me, and that's why the SH series always got me, they always did an excellent job of atmosphere and robbing you of your sense of what is supposed to logically come next.
also spongiform material (organic-looking surfaces with large open pores) grosses me out hugely. though irdk why.