A lot of people have attempted to define what makes a story speculative fiction. More useful, perhaps, is the attempt to define what makes successful speculative fiction. To my way of thinking, a simple starting point would be to ask yourself if your story has the Three C’s: Concept, Character, and Conflict.
For science fiction and fantasy, “Concept” is where it all begins. It’s the “What if?” that separates the reality of the story from our everyday reality. It can be universe-altering or it can be slight, but if the reader can’t identify what is otherwordly about your story, they likely won’t even regard it as speculative fiction. When I ask most readers what a particular SF story is about, the concept is usually what they relate first.
Concept is so important that it’s even possible to have readers accept a story as good science fiction if it’s the only one of the Three C’s present. The concept is that an empty alien spacecraft crashed outside detroit and the story is 3,000 words describing it? No one’s going to argue that it’s not science fiction. And if you’ve written a compelling 3,000 words you may even get it published. If you’ve got a strong reader base that loves to nominate you for awards, it could even become an award-winning story. So put some serious time into the concept. Think it through. Consider its ramifications. How will if affect the people in the world you’ve just created?
Which brings us to the second C. I don’t think any story ever holds up without attention to character. The people in your story need to be more than 21st-century avatars meant to ferry the reader around your concept. They are real people, affected by the world they live in, with authentic hopes and fears and perspectives. Yes, it’s possible if you’re good to let the planet become a character and hang the novel on that, but in general readers want to care about what happens to a person — not necessarily a human — in your world.
So consider the characters. Are they believable? Are they interesting? I don’t think they need to be likable per se, so long as I care about what’s going to become of them.
Of course, the easiest way to get readers to care about the characters is with the third C. Conflict is what drives your story forward, sets up the change that occurs as your words flow by. This should be organic to the concept and the characters, not something artificially imposed because you knew there should be conflict. In other words, don’t just set up hoops for your characters to jump through. Make sure the characters need something, and make that something difficult to obtain or achieve. The conflict can come from other characters, from inside the character’s own psyche, or from some external force, but your characters must be working to overcome it.
And, really, that’s all there is to the Three C’s. If you’ve got a strong concept, compelling characters, and appropriate conflict, you’ve got a rock-solid framework to hang your prose on. And that’s the foundation of a good science fiction story.