The most important aspects of a great horror novel are plotting and characterization. Does every writer have to plot their novels? Well, not if you're way smart and really know what you're doing. I learned to though, because my first novel went all over the place with weird scenes just placed in there to shock. I learned to outline so I have a sane assemblance of order as far as the events. Every scene has to relate to the plot or there'll be parts of your novel people don't care about.
The most important aspect, however, is characterization. You have to be able to get the readers to sympathize with and fall in love with the characters, especially the protagonist, so that when scary things happen to them, you're shocked and afraid--right where the author wants you.
Ray Garton's really good at this, getting us to love his characters so much old formulas become fun to read. So is Gary A. Braunbeck, making the ghost story brand-new. I can only hope to ascribe to this level of greatness.