An open question about speculative fiction.

In another post, I ventured a functional definition of speculative fiction. I said that a manuscript is "speculative" (i.e., fantasy, sci-fi, or anything in between) if it requires the writer to invent a rule or condition for their world that acts as a metaphor for the novel's theme. In other words: If you make something up, that something has to offer the reader a clue to what the book is about. Otherwise it's gratuitous.Yet, I am reminded of Occam's Razor, which says that the simplest explanation for anything is usually the truest one. The problem with my earlier definition is that it doesn't always work; there are lots of books about dragons, fairies, and/or outer space that use speculative elements just for the fun of it. Some readers like to read about dragons, for instance, so a market exists for writers who enjoy telling dragon stories. Simple as that. There is no rule that says all dragon stories must be important social commentary.So let me try a simpler definition. Where all fiction involves five basic elements--premise, theme, voice, character, plot, and style--speculative fiction also involves a sort of sidecar to premise: the concept.So, if premise is what the story is about in a few simple sentences, the concept is the invented-but-believable element that separates the story world from reality. The concept could be anything: vampires and why they exist (Interview with a Vampire), a medieval world inhabited by dragons (The Dragonbone Chair), a future America in which fertile women are required to reproduce (The Handmaid's Tale), or a alter-reality in which Irish immigrant spirits are at war with Native American spirits (Forests of the Heart). If your novel uses a concept, then it has a speculative element. Simple as that.So, here's my question. What is the difference between realist fiction and speculative fiction?And a bonus question: Where is the line that separates books shelved in a store's "general fiction" section and its "sci-fi and fantasy" section?