The mark of science fiction, its greatest gift, its handsome scar, its unalienable right, is its ability to be the window to a world of what-if. Without this you're just writing a fantasy with lasers and spaceships.
I don't care when you set your story, but if you ask the question "how would life be different with this, or that" then you're writing science fiction. More power to you. Does Star Wars show us how different the universe will be with the advent of alien creatures, or spaceships, or laser-blasters? No. It's just another adventure. You could do a scene by scene swap with Nazis for storm troopers, and jeeps for spaceships, and pistols for lasers, and get much the same story. What about the force, you ask? What about it? Does it change anyone's life? Does it reshape the universe because it exists? Does it make someone view the universe in an entirely new way? No. We see it being used, but it's sort of left by the wayside to make room for the story, and what the characters feel. The force ends up being, at best, a tool used to examine Luke's passage into the role of hero. There's a hundred coming of age movies out there that we've seen the same soul searching in, without having the resort to using the force. At its worst, the force is just a gimic for special effect (what, Obi Wan really needed to use to force to distract the guards by the tractor beam? The same thing couldn't have been done with a thrown pebble?)
Don't get me wrong, the movie's a fine story; captivating characters, lovely action, and wonderful effects, but it's a fantasy, not science fiction.
My point is that there's this great gift that science fiction brings... to be able to let someone see what a person, a culture, a world would be like if something that doesn't exist does (or if something that does exist doesn't).
Take Larry Niven... can he write characters? No, not to save his life. Can he write action? No, not so much. But can he write science fiction? Yes. Yes he can. Look at his teleportation booths and flash floods (and don't we see this every day with the Slashdot effect, or any site that suddenly gets hammered by sudden portal-link fame?) Look at organ-legging and the society it produced. Look at TASPs and the effect they have on society (certainly more effect than the Force did for the Lucas-verse).
The story doesn't have to effect an entire world. It can just as easily encompass a small cabal, or even a couple of individuals, as long as the addition of some element changes the universe they live in. As long as there's something in the story that makes them react as no human has ever reacted before.
Take Frankenstein and his creature, a story loaded with characterizations and gothic-y torment-y goodness, all qualities of "literature". Yet you've got the addition of abandoned animated flesh, and a scientist horrified by the life he's created and a refusal to adopt it. Dr. Frankenstein goes off to the embrace of his own god-given family and turns his back on the wretch he's spawned. You've got the creature, left to examine its own state of existence, its miserable alienness and aloneness, and then choosing its inevitable inhuman path, to resort to force and brutality, a dominance over the lesser lifeform (man). You also have the doctor, a coward, who at least learns from his journey into knowledge man was not meant to know, finally standing firm to not propagate the plague he's unleashed.
I'm just getting tired of people claiming that something is science fiction just because it's got a nifty new car in it, or a zap gun. Science fiction is a right, don't make me take it away from you.