Ok, the very very simple version: Science fiction is set in the future. Star Trek is deliberately set in the future. Star Wars is deliberately set in the past ("A long time ago") and therefore is something other than science fiction.

Then 20000 Leagues Under The Sea is not science fiction? (the setting was contemporary art the time of writing)

Future technology. When they set foot on the nautilus they set foot into a possible future. 20,000 poses the question of what might happen should that sort of technology be developed. It was very predictive of the power that such technology would place on a single man. Nemo is latin for "no man", telling us that no man should have such power.

I don't think you'll get very far classifying space opera as opposed to science fiction, rather than a variety of it.

It's a taxonomy which is cladistically nonsensical, which you can prove to yourself by browsing the Hugo and Nebula awards.

What about time travel, or alternate history? Is that fantasy?

edit: If you count time travel as fantasy, please do so for any SF which features FTL :-)