There's a practical definition ("Whatever that individual bookseller puts in his or her Science Fiction section"), numerous academic ones, an evasive one ("Genres don't exist!") and a sort of commonly-held gut feel ("I'll know it when I see it. Also, rockets.")
My own background is in marketing, so let's approach it that way: if "Science Fiction" was a product that we wanted to sell (and being evangelical readers, we do), what distinguishes it from the competition? What are its emotional and rational benefits?
Thinking about what makes "Science Fiction":
- It tells stories that are not bounded by time or space. (Is that distinctive compared to fantasy?)
- It is, as a category, self-reflexive. In that, SF refers to other SF, plays off of other SF, talks to other SF. (Is this distinctive? I think SF does this more than fantasy, westerns, crime and [probably] literary fiction, but what about romance? Horror?) (Is this a benefit? Arguably, to a certain type of reader - it means that your appreciation with SF is a positive feedback loop - the more you read it, the more you appreciate it.)
- It over-indexes amoungst young readers (A simple fact of the genre, although also a potential red herring due to the casaulity question. Do young readers gravitate to SF because there's something in SF that speaks to them? [If so, what? That'd be an important benefit to note.] Or does SF gravitate to young readers, and this is just a reflection of a commercial strategy? [Which is also easy to believe, SF sells to kids because... SF tries to sell to kids.])
- It extrapolates. (I'm taking a plunge here, but isn't that what SF does? In the setting, characters or plots, SF takes a something or somethings and extends it. Either logically or illogically. The something could be a technology, a social policy, a trend... I'm not sure that something is necessarily even vital to the plot, it could just be an element of the setting or whatnot, but... this feels like something SF does that no other genre does. [Except maybe horror? That is generally based on taking a fear or concern and then extending it? Discuss?])
Those are four starters... but what do you think? What distinguishes Science Fiction from any other genre? Feel free to use quotes, numbers, theses and marketing wak-wak in your answer, but it'd be best if you just said what you thought and in your own words!