Quote by Just_A_Guy_You_Know
There are a number of redundant, niche categories that could be culled, or at least streamlined. God knows why we thought 'historical' had to be its own category, like nothing turns me on more than reading about Ancient Rome. π Steam punk, horror, monster sex, mind control, supernatural, fantasy and sci-fi should really all be rolled into a single category. I think the reason we don't do it is a bunch of people who got an omnium badge would whine that we're diluting their accomplishment. But these categories really serve no purpose on the site, and just clutter things up, in my opinion.
Some would argue categories should be scrapped entirely in place of just tagging everything accurately. On some days, I might argue that. Or just a few very broad genre categories with tagging for everything else.
The fact is, the categories here (and SS isn't much better) are not a well-designed hierarchy like library subject headings. No one sat down and figured it all out. Instead, they've been tweaked and changed and added to over time due to demand and owners' whims to the point where they are almost just a more important tag. I mean, just the fact that you have some based on form (e.g. Micro, Flash, the two Poetry categories), some based on genre (like the Horror and S-F that you mention), and some based on erotic content (e.g. I.F., Voyeur, Watersports, etc.) is a sign of a messed-up subject classification system.
However, I don't think there's a will or really a need to mess with it. It is what it is. A fight over changing them isn't really a productive use of the site's time and I say that as someone who, if he was spinning up a writing site, would definitely do it differently than here or SS. I would probably have genre, form, and content as separate groups of tags and just forget about categories. Provide a tag cloud or similar for navigation purposes along with a well-designed search.