It’s always annoyed me somewhat, in a bookshop, that sci-fi hasn’t been more sub-divided. And having fantasy thrown in as well just really screws things up.
The reason behind this, is that science fiction isn’t a genre, in the way that ‘thrillers’ is. It speaks about the setting, and not about the story. Ditto with Fantasy. Of course, these days, you also get urban fantasy, which is often separated off. Probably can thank Twilight for that, much as I’d hate to give it any credit.
I guess, really, this is where the internet can come into its own, in a way that bricks and mortar stores would find really difficult to compete with. At least not without a card index 😉
Tags are perfect for this kind of thing. You can instantly create any subdivisions of a genre, and go cross genres with ease, allowing for an easy listing of, say, #military, and #scifi. I guess I should probably go and properly tag the few reviews I’ve done.
I do wish, however, that Amazon has a decent tag cloud on its books. I guess the problem would be populating it. maybe a few meta-tags too. like #popular