Twin genres! Tied together!
I don’t quite divide them up the way lots of people do, by tropes. You know what I mean: SF is spaceships and future stuff and lasers and ~technology~; F is magic and impossible creatures and the medieval-esque.
Interesting fact: ‘technology’ isn’t about machines. It’s not about iWatches and LightSails. Technology is process. It’s algorithms. It’s knowledge. The 3-field system is technology. So is bureaucracy.
For me, ‘fantasy’ uses the trappings of SFF–spaceships, magic, impossible creatures, whatever–but those trappings don’t have any impact on the fundamental human condition. There may be miracles and wonders but they either mostly address other miracles and wonders, or they provide, at best, a short-term solution. Fantasy is about coming to terms with what is unalterable. Fantasy brings you courage to face the ordinary hidden in the stories of gods.
In fantasy, when magic could save somebody’s life at the expense of changing them forever (or even without consequences), they instead come to terms with their death, because that’s what we all have to do, eventually. Magic restores the natural order and the most important magics do things like bring hope, or peace: achievements we can all strive for, right now.
In science fiction, the tropes do change things; science fiction explores how. The trappings aren’t miracles or wonders (except maybe at first). They touch many people’s lives and people respond and adapt to them. Whether the magic is sorcery or pocket computers, society adapts to it. It may become something strange or alien, or it may be just a little bit different from what we know. But the tropes aren’t just trappings. They’re (at least partially) the point.
In science fiction, when magic could save somebody’s life at the expense of changing them forever (or even without consequences), many people will face that choice. Some people will choose life. Society will change. There is no ‘natural order.’ (Some of) what is done is nothing more than thought experiments, beyond our current reach. But the ‘science’ in science fiction is methodology. Questions are asked and answers are proposed. Ideas are applied to a world.
Even defined this way, many stories straddle both fields, of course. But a lot don’t. And sometimes fairy stories are science fiction, and we all know science fiction that is really fantasy.
I think it’s an interesting enough way to approach how I read and how I write that I wanted to share it. 🙂
Dreamfarmer 
I wonder if fantasy is an inherently conservative genre, where a return to the status quo established in the bucolic first chapter before the Evil Threatens The World is always desired. In that context, world-altering implications of setting developments *must* be rejected, because they’d prevent the return to status quo.
So the elves have to go to the west, Arthur has to die, the big super magic has to destroy all magic everywhere, and so forth, because otherwise we wouldn’t get the conservative ‘return to better days’ outcome.
(tangentially related to the conversation we had about conservative scifi, and my assertion that fantasy is inherently conservative because it is largely derived from Tolkien.)
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