your story must be believable. This can include dragons and vampires and monsters but it must allow the reader to insert themselves as the protagonist and believe they can slew your dragon. Much of this is driven by the prose. A good writer can make anything come to life and this will be what gets your work published.
This rule also follows for non-fiction, memoirs, etc. The story must flow in such a way that the reader can see it unfolding before their eyes.
Any thoughts on believability?