We are deep in the slush...
Recently, I read some horror stories, and it strikes me that horror is very difficult. This is because in horror, in particular, the author needs to evoke an emotional response in the reader. Moreover, the range of emotions is not huge; generally, readers look for fear and/or a sense of the uncanny. I had a professor that always said horror needs to subvert readers' expectations of reality. Possibly, horror evokes shock or disgust--but since we focus on more macabre horror, these are less common in our 'pages.'
Supposedly Stephen King said, "Terror, when you come home and notice everything you own had been taken away and replaced by an exact substitute. It's when the lights go out and you feel something behind you, you hear it, you feel its breath against your ear, but when you turn around, there's nothing there..." That does sound terrifying! There's clearly a reason he's considered the master.
Horror has additional common elements, including exploration of wicked deeds, events, and/or characters. Often they contain plot twists near the end. Often they have a strong spooky tone and mood, created via vivid descriptions.
Send us your horror-ible stories! :)