The thing with King, though, is that it's not always about the plot. He does do those kinds of stories; Cell, for example, rolls along nicely, driven by the storyline. But in other books, like IT or Lisey's Story he expends a lot of writing building up the area, the history (personal history, I mean, backstory and all the little details) and of course, where I think his real power lies, creating some incredibly rounded characters. For a quick fright, a rolling read through some terrible situation would suffice. But for the deeper, sustained and lingering horror that King deals with sometimes, there has to be the feeling of verisimilitude, of characters that you know in and out and thus care about all the more, and -- something which is particularly explored in IT -- the sense of time and history, how this certain situation has been going on for a long time, and therefore might still go on long after the final pages...
But I do also agree that his 'shorter' novels are effective; they certainly do have more of a punch. I adore Misery.