Know Your Audience: Okay, this one applies to all writing, too. When you are writing speculative fiction, you need to understand that your audience is bright, curious and looking for something to ignite their sense of wonder. They like to figure things out for themselves rather than have it handed to them. You should not talk down to your spec-fic audience. They hate that. You have to go easy on the explanations of things or they'll get irritated and stop reading. I'll go into the handling of exposition in more detail in another post.
Know What's Already Out There: I suppose this is true for any genre also. You need to know what's already been done in the speculative fiction world so you're not just rehashing old ideas. Of course, there are never really any new ideas, but you have to give those ideas your own fresh perspective and you can't do that unless you know what perspectives have already been done to death. Reading speculative fiction, and lots of it, is basically the only way to accomplish this.
Internal Consistency: When writing speculative fiction, you're going to create a whole new world, or perhaps it will be our world, but with a different set of rules. Your world must be consistent with itself, or your readers will hate you. Your technology must function the same way all the time. Your magic must operate by the same set of rules all the time. Your readers have to willing suspend their disbelief, and it is hard to do that if your world doesn't abide by it's own set of rules. World building will also be the topic of a near-future post.
So, any other speculative fiction rules I might have missed?
Up next: Writing Speculative Fiction: Part IV--Exposition
Read Part I--What is Speculative Fiction?
Read Part II--The Difference Between Science Fiction and Fantasy