"And then I had an epiphany, boiling it down to five words that work for both sides of the genre: hard fantasy and hard SF alike are concerned with how stuff works, and why.
In science fiction, that can mean physics, computing, biochemistry, etc. A hard SF story is one that takes the known facts of those sciences and extrapolates them, rigorously exploring the mechanisms by which they operate, and how they might be made to operate in new, expanded ways. The equivalent in fantasy, then, is the type of work I’ve often labeled “anthropologically rigorous” — concerned with history, religion, politics, systems of magic, etc. What happens if you set your world conditions like this? Just as in SF, a given novel may devote scads of attention to one topic while ignoring others; finely-tuned interstellar travel matched with nonsensical alien biology is paralleled by, say, Tolkien, who thought through his cosmology and linguistics like whoa but didn’t seem much concerned with where anybody outside of the Shire got their food. It’s hard linguistic fantasy, hard cosmological fantasy, but politics and economics fall by the wayside.
How stuff works, and why. George R. R. Martin treats his politics with all the attention and rigor you could hope for. Jacqueline Carey extrapolates an alternate Europe where Christianity never homogenized Western culture. To a hard SF writer, these may not look like much; human culture and behavior are inescapably fuzzy, and do not lend themselves to replicable laboratory experiments, much less testable thought-experiments. But a hundred years of social science research has produced some pretty good models for understanding how people live, and I think it’s possible to devote in-depth attention to those aspects just as one can with the natural sciences.
The result is hard fantasy. And I don’t know about you, but I am a sucker for authors who write it."