There was an old rule of thumb that you could tell science fiction from fantsy by the book cover. One had a rocket ship and one had a tree.
Then someone put forth the definition that science fiction made the improbable possible and fantasy made the impossible probable.
Lately it seems that that line is even fuzzier than before.
Look at Marion Zimmer Bradley's Darkover series. It happens on another planet and deals with telepaths. There's a spaceport, so it could have a rocket ship on the cover. But it is a primitive planet with people who have pretty magical skills, so the tree applies, also.
I am reading Terry Brooks' Armageddon's Children series. It's post apocalyptical and the survivors meet elves. So that might be science fiction/fantasy/horror. The second book has a dragon on the cover. But that might be so that his fantasy fans didn't get scared away.
I think Ursula LeGuin's Left Hand of Darkness would fall into the same murky category.
I'm seeing the category Speculative Fiction as a genre in a few places. But we need to better identify our subgenres. I read a lot of fantasy and yet I rarely read any Sword and Sorcery stuff. Nor do I read anything that requires massive battles and lots of blood and gore. There are so many types of fantasy and science fiction out there, that I think we need to create new categories for the new breed of ebook buyers.