I found this idea highlighted recently, when I read a novel by a well-known horror novelist. I like his work, overall, but I find that he tends to rely on the "gross-out" factor, rather than on a solid antagonist that the reader can believe and believe in. I was reading a laundry list of atrocities - limbs hacked off, entrails left trailing in the dust, and, at one point, a woman's head was...I kid you not...inserted in her own vagina - and I didn't care. Because up to that point, the bad guys, a group of mysterious supernatural entities, had not, as far as I was concerned, been adequately developed. They were just vehicles to move the story forward, and it wasn't enough. Fortunately, the final battle between the forces of good, and these evil beings made up for some of what I felt was missing, but do you want to take the chance that your readers will bail on your story out of boredom? No, I didn't think so.
Look, clearly there is a market for blood and guts. "Splatter Gore" has been a respected sub-genre of horror for a very long time, and there are writers in the field who write as well as anyone. But there has to be more to your story than hack-and-slash. Your job as a writer is to make your reader forget that he or she is reading, to become so involved, that the words on the page disappear, and the story comes to life in the mind.
If the reader is 250 pages into your work and still waiting for something to happen, then you're not doing your job.
Words to write by...