|Some real SF in this one|
"What if?" makes it science fiction.
If a story doesn't ask "What if?" it isn't science fiction, no matter how many hyperspace jumps it takes or alien telepaths it includes. Instead, it is science fiction's prettier, somewhat dimmer cousin - space opera. (Or something with "punk" in its name. Don't get me started about that.) People associate space opera with blazing blasters, robots galore, fifty-eight different alien species on any given planet, and vast galactic empires. But really, space opera is mostly just science fiction with the extrapolation removed.
Does this difference even matter? After all, genre classification is a tricky beast, and subgenre classification is a super-evolved chameleo-mutant. It's the stuff of endless, tedious late Saturday night nerd-offs. The eyes glaze over, the head droops, and before you know it, you're in an alcoholic torpor as two sophomores with skin problems thrash out the subject until they close the Loscon hospitality suite.
But if you're a spec-fic writer or other storyteller, it is important, because most of what people now consider to be science fiction is actually space opera. Star Wars, Star Trek, Babylon 5, Doctor Who - they're all space operas, instead of straight-up science fiction. If you're going to pitch either genre, you should know the difference.
Obviously the same writer can write both. And some franchises, like the original Star Trek, veer from one to the other and back. When Heinlein wrote Methuselah's Children, he was writing science fiction - even if it was also adventure fiction. ("What if civilized human beings suddenly discover that their next-door neighbors will live five times longer than they? Will their civilization hold, or break?") When he wrote Double Star, that was space opera (but a lot of fun). Maria Doria Russell's superb novel The Sparrow is pure science fiction, even though advanced technology barely makes an appearance; Lois McMaster Bujold's equally terrific Vorkosigan Saga is almost pure space opera (with one or two exceptions, like the social structure of the planet Cetaganda).
Space opera is fun; I love it. I'll probably be writing it. But the one thing it cannot deliver, that only science fiction can give, is the authentic jolt of the weird. If the story twists your brain; if it shows you a rational, utterly alien possibility based on the logical extrapolation of a premise - that jolt is the sign that you're encountering real science fiction.
Look for it. It's hard to find, but it's worth it.