Realism

A horror movie with too many fictional elements is easily forgotten. Those just aren't as impactful as ones that showcase how depraved reality can be.

That's why I personally find movies such as "The Thing" as top tier. You can really see how close to reality that movie is. Although they seem to have downplayed certain elements in order to appeal to a larger audience, what they presented is as close to reality as it gets when it comes to movies.