You know what kinda irritates me?
When the writer(s) go out of their way to explain everything in a horror story. To me, it makes the story less effective. Part of what makes horror stories scary is that, for the most part, the main characters don't know what's causing the scary shit to go down. Slenderman is a good example, we never learn exactly what Slendy is or where he came from, we just know that he's here and shit has hit the fan. Explaining everything about the monster in a massive infodump just makes the monster less scary. What a horror writer should do is leave little bits of information around to to hint at the nature of what's going on while still keeping most of the mystery.
So what are your horror pet peeves?