It is no secret that so-called "speculative" fiction--horror, fantasy and science fiction--is generally regarded as "lower grade" fiction. I know many avid readers who refuse to touch it, in fact.
The problem is not that people aren't open to speculative elements. Most readers, in fact, are open to speculative elements in their fiction. They've been trained to be open to it, in fact, ever since their first grade school and high school literature classes.
Speculative elements can be found throughout the body of Western literature. Consider, for example, the ghost of Hamlet's father and the witches in Macbeth. A fair number of "classical" literary works contain fantastic and supernatural elements.
The problem, rather, is that so much of modern speculative fiction is simply poor writing--plodding plots, wooden characters, and repetitive themes. (How many teenage vampires have we see in books and film in recent years? And how many more will the publishing industry throw at us in the years to come?)
But what about Stephen King? Here is a horror writer who has developed not only a cult following--but who has also made inroads among general readers. His sales numbers prove as much.
Stephen King succeeds so lucratively not because his stories are particularly "frightening". There are a few spooky moments in King's assorted novels, but nothing to compare to The Exorcist. I've never had a sleepless night over a Stephen King novel. I suspect that most of his readers would report the same.
King finds a wide readership because he creates compelling characters with whom readers can identify. His plots, moreover, involve basic human dilemmas that are not far removed from the experiences of everyman and everywoman--even if they occur against a supernatural backdrop.
When writing speculative elements, it is important to create compelling monsters, supernatural villains, and other worlds. But never at the expense of creating a compelling story.