I propose that worldbuilding is the primary distinguishing characteristic of SF and fantasy (at least at a superficial level). Get the worldbuilding wrong, and your readers won’t be able to get a grip on the story line or the motivation of your characters. Or worse — they’ll get a grip, and realize that your story is, at best, a western or an age-of-sail yarn with the serial numbers filed off: that the trappings of the fantastic are only there to add a spurious sense of exoticism to an everyday tale.
Charlie Stross via World building 101 – Charlie’s Diary.