Sure, I'll be glad to throw a little light on the darker corners.
We'll start with a basic understanding of some terms, but first I have to begin by saying that I disagree very strongly that BDSM is just another fetish. And understanding that (and a few other important details) may help some of you make more sales.
BDSM can indeed contain many elements of fetish behavior but so does most of life. However, there's a very important distinction to remember, and that's the fact that fetish behavior is centered on an object or an act (yes, even partners can be objectifed and depersonalized in fetish play, but that only helps clarify things). BDSM is all about the person/people involved. Sure, there are all sorts of tools and toys used in BDSM play, but the ultimate focus is on the people, not the things or the acts. Power is exchanged between people, submission is given to a person, not to a thing or an act.
Okay, let's look a little closer...
BDSM - this is a sort of catch-all term that's probably the best choice if you only are going to have one category for all of "this kind of stuff." BDSM breaks down as B&D / D&S / S&M so it encompasses Bondage and Discipline, Dominance and Submission, Sadism and Masochism. However, there are no intrinsic dividing lines regarding sexual preferences, which is one big problem when using "BDSM" (or "Fetish") as your catch-all category on a Link List or similar site. More about that in a minute.
Fetish - here we're really referring mostly to sexual fetishes since that's what we're selling most of the time. So if we're talking about the "things" which cause sexual arousal in our audience, then we're in the realm of sexual fetishes and paraphilia. The things may be actual inanimate objects (high heels, rubberwear) or body parts (big tits, feet) or even situations/acts (locked in a dark closet, boot worship, etc). So again, many Link Lists use this as their catch-all category of all kinds of kinky sex. It also lacks the sexual preferences divisions that are really needed though.
When we're talking about breaking things down for categories on a Link List a lot of what applies for those in the real BDSM Lifestyle or the Scene (capitalization intentional) is less important, unless of course you're actively trying to market to those people, which is really something best left to those who are in the Life themselves (seriously, there's no way to fake it well enough to make any money that way).
So for most readers of this board what we're interested in is "BDSM and Kinky Sex" and we're aiming at the surfers who find that arousing. BDSM Lifestyle surfers are a whole different breed and there are far less of them so it's a smarter move not to focus on them unless you're one of them yourself anyway.
Let's get down to the important stuff. One of the more critical things to understand is that the dividing lines of sexual orientation have to be considered when making your categories. See, I told you I'd get back to this.
Consider this as the owner of a Link List or some niche hubs: do you actively promote gay paysites to your surfers who've already told you they're straight? I mean by having banners and buttons and photos of gay sex all over your pages aimed at straight surfers? If you have a 'blowjobs' category page and the text link says: "Deep Throating Facial Freak - an awesome deep throating sequence ending with a hot facial!" - should I have any concern about seeing some dude blowing another dude when I click that link?
If you don't understand why you normally wouldn't mix gay and straight blowjobs on the same page you really need to run some tests. But I think most of you know that it's better to keep them separated for marketing reasons alone. So if you're not already doing the same thing with your Fetish and BDSM category pages, I promise you that you're losing signups that you can get with better filtering.
While I disagreed with Hinc about BDSM not being a fetish, I do agree that what's really needed is a tagging system to go along with categories. And if you're already using tags on your site, you can expand on what I'm suggesting here. But if all you have is categories you can still improve things.
If you only use categories you can just split all FemDomme listings into their own category, and if you have enough listings, then you can split FemDomme into FemDomme/malesub and FemDomme/femsub (also called LezDomme).
There are other distinctions you can make if you have the listings for it, for example in FemDomme/malesub there are men who want to see the guy getting strapon fucked by her and others for whom that ruins everything. And for those for whom it ruins things there are those who are just turned off by strapon sex and others who are disgusted by the very concept of a FemDomme having any sexual contact with a malesub. So a category for FemDomme/strapon is a good idea (and divided between malesubs and femsubs).
But at least start by taking FemDomme out of other categories and you'll make a good start. And split that into malesub vs femsub when you can.
Leave BDSM for maledom/femsub. Maybe even call it BDSM/Maledom. And then start shaving away at that one too. For example, if you have enough listings to break spanking into its own category, do that. But only real spankings should go there, not spankings mixed with other things. Bondage can also have its own category if you can fill it, and again I mean the focus is on the bondage itself, not on other things.
Bondage as a Category - Please don't make the mistake of using this as your catch-all category. That's a very bad idea. Bondage is one small part of BDSM and can be involved in some fetishes. But it's definitely not a catch-all for other things. If you use a bondage category the focus should be on the ropes and the rigging, the chains, manacles and handcuffs, the bondage wear and bondage gear. It's the actual bondage that bondage fans want to see (and it should be good bondage to appeal to those surfers). Sexual acts can be included but the focus should be more on the bondage than on pain created by other activities.
Fetish as a Category - While many sites use this as a catch-all category the way some use BDSM as their catch-all, it's really not a good idea to put all the listings in this one either, for all the reasons already mentioned plus the fact that while fetishes could be in the BDSM category, BDSM really should not be presented as a fetish if you want to sell it. And you can sell fetishes much better if you present the major fetishes on their own pages. (I think I'll let someone else tackle what the major fetishes are, or I'll make a run at that later.)
Anyway... that's the really important part, the selling. If you want to get more signups to BDSM and Fetish paysites then you need to first filter the traffic to more finely-tuned category pages and then present very specific ads and promotions to the surfers visiting those category pages. And speak to them with words tailored to their specific interests which lead them to those pages. (For example, on your FemDomme pages you'd never refer to them as 'kinky bitches' or 'hot latex sluts' unless you want to lose your surfers.)
So let's do a quick recap of the basic categories you should use if you have the listings for them, or at least what you should be planning for as you grow:
BDSM
BDSM/maledom/femsub
BDSM/maledom/malesub (usually but not always gay)
BDSM/FemDomme/malesub
BDSM/FemDomme/malesub/strapon
BDSM/FemDomme/femsub (LezDomme but also not always gay)
BDSM/bondage
BDSM/spanking
Fetishes
(and all the sub-cats you can make, focused on the 'things')
Okay, that's all for now. Feel free to hit me up on icq or catch me in the GG&J chat most days if you have questions as you're working on your category pages. And of course you can post questions right here too.