A very good question. Eric Rothkirch has some answers over on his blog. I think Isaac Asimov once said that science fiction can be defined as literature about the changes facing mankind, and while I think this is a little grand, I still take pride in telling people that I write science fiction. Ken Macleod, however, told me that Déjà Vu is not science fiction — and I think he’s probably right. Science fiction must be contemplative, somewhat in-depth, and should push ideas to the forefront. I’m happy writing about big ideas — artificial intelligence, for example — but I also feel that stories about people, not ideas (though the ideas might be the strongest aspect of a piece of fiction), are the most compelling.
Anywho, Eric’s post struck me as interesting, and you might want to check it out.