I like the answer provided by Robert Beveridge,
https://www.quora.com/What-makes-a-good-horror-suspense-story
In a nutshell, he says the key elements are pacing, characterization and the need to give all the clues but maintain the mystery until the final reveal.
Pacing is achieved through the use of shorter, simpler language and word choice.
For a story to work, the reader needs an attachment to the characters, good or bad. Elements of that character acting like a real person bring the story to life and attach readers to characters they relate to.
The ending can’t be obvious or the terror flops. Writers are able to maintain the mystery by tossing in red herrings that send readers in the wromg direction.
JohnMatsui.com 