I'm not impressed with some of the slush horror I've seen. Maybe 'cause it's e-z to write bad horror. Just write a story with lots of people getting killed by someone or something bad. Yawn. If the story really sucks, I end up looking forward to the protagonist getting put out of his/her misery.
So, what are the keys to good horror? Well, ya gotta establish an interesting protagonist. The story can't be about horror; it has to be about an interesting character experiencing horror. A well-developed character can make even the most trope horror plots (i.e. murderer on the loose, monster attacking stranded group, angry ghost seeking revenge, etc) interesting. Even better, avoid trope horror plots altogether. Erase all those bad horror movies you saw as a teenager while trying to round the bases with your date and start from scratch.
If you submit a story that manages to do both of these, I'll make those Electric Spec editors publish it by biting their noses until they agree.