Nicked from Grits.
Creatures/Aliens
Dude, I write Doctor Who porn. Next question.
Mpreg
Jesus fuck, no. Just no. Considering I find regular RL pregnancy a major squick as the sort of body horror I never ever want to go through, and the amount of disgustingly sugary fluffy "kids are the culmination of love and make everything wonderful" crap you get in pregnancy fics... I... just... no. Seriously, no. This probably my biggest squick.
Slave, BDSM, D/S, S/M
Bring it on. If it's well-written, that is. All of those are hard to get right and can get very cringeworthy and OOC. Don't even get me started on most BDSM fiction written by actual practitioners, because so much of it is so bad and says some really depressing things about the people involved. I like my BDSM to be intellectual, thank you.
Age Difference
Not a big deal.
Bloodplay
In controlled amounts, if there is actual proper sex involved. I don't really get off on a scene if it's just torture and with no actual fucking or release in it.
Underage (chan)
Eh, no. I'm not particularily aroused by prepubescent people. Now, I'm pretty sure Theta and Koschei did some naughty groping under the sheets in the dorm when they were young, but I wouldn't write that as a pornographic scene to get off on--I'd mostly mention it happening, gloss over it and move on to the time when they're actually a bit older, late teens and stuff and *then* get into the dirty shagging. It'd be hypocritical of me to say kids only develop sexualities when they hit that magical legal age (I was humping every object in sight by the time I was six), but I wouldn't want to write porn about it. I write and read about characters I respond to sexually, and I don't respond to kids.
Crossdressing/Genderbender
Oh yeah, baby.
Bodyswap
It's not a major kink of mine, but can be fun if it's done right. I don't seek it out, but if someone writes one that makes me laugh, sure.
Dubcon
I tend to go for love-hate pairings, so this kind of happens all the time. Especially the good old "oh, fuck off, don't do tha--oh, bollocks, ok, I am enjoying it anyway" type of thing. Because those characters tend to be stubborn bastards who are in denial.
Noncon
This really depends on how you define it. If it's a true rape scene, then it is fucking horrible and squicky. If it's seriously meant to titillate, then it's not the same thing. So it's kind of hard to draw the line there. A horror story where things get really fucked up might have a noncon scene and it might work in the context. But I don't really seek out fics where you're supposed to get off on the violence. I still shudder at the memory of that one terrifying B7 fan woman for whom mutilating Blake (and only mutilating him, through some OC or another) in some incredibly creepy stories was genuine porn (and the woman seemed pretty psychopathic herself, so it was pretty scary).
Incest/Selfcest?
Again, I write Doctor Who porn. This is a show where selfcest is pretty canon (if it wasn't after the world of Masters, it certainly became that after Amy/Amy)...
Threesome/moresome/gangbang
Oh yes. Often this works best as crack, though. Or just PWPness. I tend to prefer just twosomes because then I can focus on the character dynamic I get off on in the first place, but I won't say no to a bit of threesome action sometimes. It really depends on the story. I wouldn't be all that big on a threesome if it was in a really, really emotionally intense story that was focused on two specific characters. It might feel really tacked on. But again, there are some brilliant writers out there who can write really great threesome fics--either love triangles or outright polyamorous fics. So it's hard to say. Mostly I tend to prefer threesomes in PWP form, though.
Death/Snuff fics
I don't seek it out as a kink, but I've written it--I don't know if I've written it in the *snuff* sense, but I've written it in the horror sci-fi sense (all hail
the Sickathon!) and as
tragedy. Not sure if I'd go for random deaths for the hell of it. It'd still have to be tied to the characters (in the above cases, one is to do with the Master at his most psycho and Timeys being able to regenerate, and the second one is about how Handy pretty much fails at being a human and needs to be released from his misery--and even then, it's not a death fetish fic, it's a compassionate and merciful fic).
Watersports/scat
Not really my thing. I do like anal-oral and enemas after someone's already been cleaned up, but the whole point about that is that there is no actual scat involved. Sort of like putting a knife to someone's throat would be hot but stabbing them wouldn't be. Probably because you take something dirty and then make it clean and suddenly you have this forbidden territory to play with for a limited amount of time, and that has its own fascination. Clean bums are wonderful. Scat? Not so much.
Crossovers
Depends on the fandoms. This mostly works as humour, again. Although I've read some good, serious ones. And if we're talking Who, the show has always been merrily jumping from one genre to another (you get historicals, you get hard SF, you get pulp SF, you get murder mysteries, you get Gothic horror, you get crackheaded comedy, etc... sometimes all in one serial!) so that kind of tends to accommodate crossovers better.
Unhappy/wide-open endings
Sure, if they are well-written. I actually prefer open/sad endings to fluffy happy ones if the fluffy happy ones seem too corny. Look, I'm a Blake's 7 fan! I thought the end of LoM actually pulled some of the punches when it didn't end at Sam's jump! I do like some happy endings sometimes, of course, especially if I'm looking for something comforting and fluffy. I think I've come to enjoy them more the older I get.
Violence/torture
See some of the above answers. I don't tend to go for torture for torture's sake. But if it's well-written, in character and there's context for it and it isn't the whole point of the story, sure.
Established relationship
I really, really, really don't go for buddy ships or buddy fics or most canon ships. I have trouble understanding them, personally--if there's no conflict and they're already cuddling, I don't really feel moved to write it or read it. And if something really cuddly and lovely breaks apart and there's a breach of trust and some really lovely characters get abused, it'd just make me miserable--I tend to favour angsty ships with messed-up characters because they a) start from a point of conflict so there's actually no trust to breach, and if you're lucky, there might actually be a temporary truce or healing and b) they usually deserve each other. But then again, I've read pretty cool established relationship fics with some of the love/hate pairings I tend to like. I do tend to prefer 'ships where it takes a lot of effort for the characters to click (even if they're in the middle of hard fucking, they might still be grumpy at the other person). I feel there's more of a journey to be had there. Drama emerges from conflict, as they say.
And that just makes me want to doodle the Doctor and the Master shopping for curtains. And the Master buys a pair of evil curtains. Look, this is the man who designed a killer armchair in his very first story...