Athelstan, who is so far my most useful foil, raises an interesting point. Can SF and Fantasy be seperated into two distinct genres?
I think they can, and, unlike SF, it can be boiled down to a single element: magic. Not monsters (for what dragon or vampire can compete with SF's strangest aliens?), nor plate armour or plots, but magic. It is the presence of Gandalf and the other wizards and witches that make Middle Earth a fantastical text; similarly it is the blood magic in George R.R. Martin's Song of Ice and Fire series that places it firmly in the realm of the fantastic.
For in SF, no matter the background level of technology, no matter the plot, there is nothing that cannot be explained by some form of science - be it handwavium or the hardest of hard science. But magic; oh! magic is the realm of the unknowable, the spirit world, the darkness between realities... and it is a fantasy. Real magicians are tricksters, who lie and manipulate - often with great skill - for our entertainment. The magic of, say, LeGuin's Earthsea or Terry Pratchett's Discworld, defies analysis. It simply is, and often comes with gods and demons.
Now, of course, as has been said, "any sufficiently advanced technology is indistinguishable from magic", and the Time Lords of Dr Who were a Type IV Civilisation as stated by the Time Lord Marnal: "We had no equals. We controlled the fundamental forces of the entire universe. Nothing could communicate with us on our level"
But these gods are technological gods. Their magic is science.
The reality is, then, that Fantasy, unlike SF, relies on magic - be it ever so little - to root it in the genre of Fantasy.
Apologies, by the way, for the shortness of this post and the lack of an update on Wednesday; I have been a bit busier than normal at work and am as a result less inclined to do much more than relax with my family and then collapse into bed...!