I bet it's something scary. For me it's the face of Pennywise. When I talked about John Grisham on Monday I mentioned how some writers are put into boxes. Grisham is in the lawyer box. King seems to have been placed firmly in the horror box.
It is a label that is meant to diminish his abilities as a writer. And many consider him to be not very good. Harold Bloom famously decried his National Book Award Lifetime Achievement by saying, "The decision...is extraordinary, another low in the shocking process of dumbing down our cultural life." The full article is amazing for it's hatred. He goes on to say that King is terrible on a word-by-word basis.
Bloom's screed led Orson Scott Card to respond with:
"Let me assure you that King's work most definitely is literature, because it was written to be published and is read with admiration. What Snyder really means is that it is not the literature preferred by the academic-literary elite."
I agree with Card. King has consistently proven to be a Big Name author. He sells a lot of books, but is clearly outside the box of literary fiction. Personally, I find these distinctions to be silly. My main concern is always - Do I enjoy reading it? And I always enjoy King's work.
He continues to push the walls of the commercial box outwards. He experiments with the form and process of getting the book to his readers frequently. In 2000 King was the first Big Name author to make a book available exclusively on the internet. His novella Riding The Bullet was sold for $2.50. He also published a serialized novel The Plant on his website the same year. In a nod to pre-20th century publishing, he published The Green Mile in 6 parts in the summer of 1996. In 1999 he published Blood & Smoke as an audio book.
Many would say that these things are gimmicks, and they are, but these are the types of things that make King interesting. He wants to deliver a good story and also to package that story in an engaging way. He is the type of author that keeps people interested.
If anything I'd compare him to Philip K. Dick. They both use genre to reveal character. The fantasy/horror elements are there to show the truths of the people involved. To say something broader about the worlds they live in. Both are also often underestimated as writers.
One final note: King's books are all interconnected. He has spent a huge portion of his career tying his novels into his Dark Tower series. The threads connecting the books are legion.
If that idea blows your mind a little, it should. He's worth another look.