I tend to agree with Scummy. Of course that's not to say that SF can't be Big Idea capital L literature (The Dispossessed is one of the best novels of any classification I've read after all). On the whole, though, my favorite distinction came from Matthew Stover. Paraphrasing him, he said that science fiction is generally about the creative application of technology, and epic fantasy the conflicts and truths of inner self. I dug it.