Did I say seven deadly perils?

Here's one more.

Nothing kills a science fiction story faster than bad science. To a reader who knows anything at all about science, it brings the story to a crashing halt when a story reveals ignorance on the part of the author. It can be something as simple and obvious as a spaceship banking and whooshing through turns (spaceships don't bank and don't whoosh, because they're in a vacuum and have no air to push against or disturb), or a misunderstanding about some pertinent bit of biology in a story in which that is important.

The same thing is true in fantasy. You might not be dealing with science, but a lot of fantasy grows out of old stories and mythology — and if you're planning to use that, you'd better learn something about it. Magic you might be able to establish on your own, but you'd better think carefully about how it works, or you'll lose readers through inconsistency. Are you going to have people riding horses? Engaging in sword fights? Better learn something about that.