Disclaimer: Avid Stephen King fan - the guy taught me how to read!
But if you look at his horror books, there isn't anything terribly original going on in his monsters. Vampires, telepathy, ghosts, evil houses, dogs, disease - all of it's been done before. What makes the books so fascinating is the characters. He has the ability to zoom into these people, get us into their heads and make them real for us. We either see parts of ourselves in these characters - or parts of ourselves we wish we had.
Because we relate so powerfully to these people, we flinch when the vampire crashes through the window, not because vampires creep us out so much, but because these characters are ours, we're invested, and we care.
That's the mark of a true master.