In my opinion one of the problems with most horror movies of today, is that they rely too much on physical horror, rather than subject matter horror. Basically they rely too much on jump scores, and gore, and kidnap/trap situations, rather than telling an actual story with horrific subject matter if that makes sense.
So I think that the ideas are what makes a horror movie a true horror, and not physical horror as much as a lot of movies rely on, in my opinion.