John Scalzi, writer of science fiction and recent GOH at Capricon has an article about movies and science fiction, an often awkward pair. This is an endless topic among sf readers. Especially with movies, the nature of movie production tends to stomp the sf out of them. I still don’t think Scalzi really gets the meat of the argument.
The vital element in speculative fiction is that it raises interesting questions–predictions about the future or about whether aspects of our society are necessary or universal, just to pick two.
Star Wars is space fantasy because it eschews sf and tells a fantasy tale of adventure and superpowers. The space future setting doesn’t make it sf any more than it did for Bugs Bunny cartoons with Marvin the Martian.
The science content is not a critical aspect of sf, but it is a signifier. Good speculative fiction respects science to the extent it can while telling it’s story. It does this so the reader or viewer has a context in which to think about the ideas raised by the work. If ‘it’s all a dream’ or ‘you’ve thought about this more than the director’ is the best answer to the questions the work raises then doesn’t work as sf.
Many movies with a futurist setting ignore all the rules of the world, violating laws of nature randomly. It’s a flag that the author isn’t telling a story you are meant to think about, just an adventure romp or a horror tale. Shows like Star Trek jump back and forth across this divide–it pulls things together for a spot of speculative fiction but then retreats fantasy.