The following post may have the possible side effect of making me somewhat unpopular, especially with those who can't tell my serious bits from my humorous ones. Luckily, I don't really care too much 'bout those people. I'm also not entirely sure where one ends and the other begins in this post to be honest...
To me, one of the major differences between Fantasy and Science-Fiction is the level of BS and how well it works in the setting you're creating. Perhaps level isn't quite the right word though. Nature? For instance...
In 9.9 out of 10 Fantasy settings I encounter, the bottom line answer for the why of all things is 'Magic'. Of course it is. It's Fantasy. At the same time it seems to me that very few writers and even fewer gamers determine what, if anything, magic is. I guess what I mean is, I feel you could put any word there and it would be just as valid (or invalid).
"How is it that these great and terrible winged lizards fly through the air and breath fire o' mighty and majestic master?", asked the young apprentice of Morbrood the Less-Dated.
"Why it's poop my boy", exclaimed the dateless Morbrood, "Everyone knows poop can do anything".
Now I know what you're thinking. Science-Fiction is the same thing. But it isn't to me.
In Science-Fiction, the BS needs consistency. It needs to make sense on close inspection, even if it's only internal sense in the setting. Just saying, "It's Science!" just doesn't cut it. In Fantasy, the cop-out is excepted, even expected. In Sci-Fi it comes off as lazy or poorly thought out.
Kind of a double standard. See, I think magic should mean something too. It should be something, even if that something is difficult to handle and impossible to fully understand. It should represent an idea or a concept on the nature of the nature of things and not simply be a word whose very definition is 'to hand wave; to not have a real answer. See also 'Full of crap.''
Not sure why this was on my mind but it was and so now, like an embarrassing youtube video of that party you went to last weekend, it's on the internet.