Innocence and isolation
I've heard it said that the cornerstones of horror are innocence and isolation. The innocence aspect is supposed to encourage audience sympathy, but personally? I know I'm no innocent and I'm not invested in protecting innocence the way, say, a parent might be.
Isolation can be physical, social, or psychological and ensures that the hero/ine faces the enemy alone. Often, they are outgunned by the villain(s) as well. This can lead to Bambi vs Godzilla syndrome, in my opinion, and solutions being handed down by the god in the machine (the author). Those aren't satisfying endings, since the heroine did not "earn" anything in the story.
This may be why I'm not a fan of horror -- on top of any additional writing problems manifesting in bad dialogue, illogical plot lines, and cardboard characters. Horror is as prone to those problems as any genre. Or perhaps it would be more fair to say that any genre is as prone to that as horror is.
A proverbial virgin being chased by a serial killer, or haunted by the angry ghost of some old house? That's just a cosmic misunderstanding. An oversized pain in the ass.
In my opinion, a dab of gore will do ya in most situations. If you've read my stories you know I'm willing to get explicit and horrible when the characters are willing to do that. Horror as a genre is a different beast, though. I'm treading closer to it than usual in my current WIP, which is turning out to be a dark fantasy.
What makes the story dark, in my opinion, is not the gruesome things that happen but why those gruesome things happen. It's also the hero's temptation to let those whys infiltrate him and lead him to begin inflicting horrors himself. The drama of resisting corruption has a particular attraction to me.
If you were going to write a horror story, how would you make it compelling to yourself? What makes you shudder?