I was thinking more about the mythos-logos distinction and its relation to speculative fiction these past few days. It occured to me that science fiction seems to focus on working out the implications of technical problems. Typical questions include what impact artificial intelligence would have on society or whether quantum physics could allow faster than light travel. More generally, science fiction can examine the future of society, our definitions of humanity, or anything else one can debate logically. Works like 1984 or the Left Hand of Darkness don't really pay much attention to technology as such. But they fall under the science fiction banner because they examine and develop premises in a "scientific" manner.
Fantasy, by contrast, focuses more on "spiritual" questions, ones that don't really lend themselves to empirical analysis. These might include the meaning of true love or the nature of divinity. One can't really analyze the temptation of evil in a scientific manner, but they can portray it through metaphor and archetypes. The one ring of LotR, for example, gives concrete form to a metaphysical concept.
As a corrolary, fantasy seems less literal to me than science fiction. The ring in LotR, for example, serves as a symbol of power rather than a concrete device. We aren't supposed to worry about the mechanics of creating evil rings, but rather the temptation of power that they embody. Most writers of fantasy don't worry much about the economic impact that functional magic would have or how the hell dragons fit into a working eco-system. I'd almost say that magic differs from physics precisely in that its mechanics don't matter but rather its thematic role and effect on plot.
This doesn't rule out the possibility of ambiguity or hybrid genres, however. Plenty of science fiction addresses moral questions that technically lie outside of science. Similarly, nothing stops a fantasy writer from examining the effects of magic on society and portraying a realistic ecology.
"There was a particular car I soon came to think of as distinctly St. Louis-ish: a gigantic white S.U.V. with a W. bumper sticker on it for George W. Bush."