A few of my preferences:
* Mermaids! I love mermaids. I've been thrilled by everyone enjoying the glimpse of the Undersea in One Salt Sea, because wow did those chapters feel self-indulgent. I could have written a whole book just explaining how the social structure of the Undersea functions. Someday, if I get a good enough excuse, maybe I will.
* Evil twins. Yeah, I know, it's a Patty Duke cliche, and I don't care. I love me some evil twin action. Blame my early exposure to All My Children and move on.
* Mathematicians and scientists in leading rolls. I think math is sexy. Science is basically my favorite thing that isn't the Great Pumpkin or my cats. It's pretty rare to find a book of mine that doesn't have at least one of these character types represented. (Ironically, Feed didn't need a scientist because I was the scientist, with all that delicious virology kicking around.)
* Alternate universes and timelines. Yes, I love breaking continuity and seeing what happens when it's put together in a new shape. Enough so that sometimes people have to hold me down and take the hammer away, since otherwise, I'll just keep smashing things. My one regret about prose as a primary medium is that it's hard to pull off alt-universes in most prose settings.
* The malleability of death. Look, I grew up on X-Men comics, soap operas, and horror movies. I enjoy playing with the elasticity of mortality, and finding ways around things that seem permanent. You can't cheat, but watching your dead girlfriend's robot replacement come to terms with the fact that she's really a brain in a jar delights me.
...there are more, but you get the idea.
One of the interesting things about knowing and being at peace with my narrative kinks is that I get much, much pickier about how they're used. You can't just raise the dead and expect me to be happy; I want it to make sense within the rules of your universe, hang together internally, and be fair to the character you've just brought back. If you're going to have a lead scientist, they'd better be a scientist, and not a magical knows-everything widget that can somehow apply every field of science KNOWN TO MAN to whatever situation they happen to be in (Winnifred Burkle, I'm sorry, but I'm looking at you).
If you're going to do an alternate universe, I expect you to think it all the way through. Yes, all the way through. One of my favorite shows rebooted their continuity two seasons ago, and while they made the usual assortment of flashy surface changes, they didn't consider all the ramifications of those changes. The fact that at least two of the characters involved didn't tear down heaven and earth looking for a way back to the original timeline was incredibly disappointing to me. (Shawn says this is because I over think these things. I point you, again, to my list of narrative kinks. These are the things I am programmed to over think!) Basically, I want stories that will give me what I want, but really commit to giving it, not tap-dance around going all the way.
Also, often, narrative kinks are a lot like salt or bacon: a little can go a long way. I adored Marvel's House of M alt-universe, but I would have been annoyed if it had replaced the main Marvel Universe completely (even though it was an awesome setting, and I want them to do more with it). I'm enjoying the current season of Fringe, with its re-imagined continuity...and at the same time, I find myself restlessly demanding the original timeline back, because I invested a lot of time and emotional attachment in those characters, those relationships, and every delighted "oh, it went like that over here" is followed by a "...wait, does that mean that this other thing didn't happen?" So sometimes, getting what you think you want out of a story isn't ideal.
And this is why I have proofreaders and editors who don't share my narrative kinks. They may encourage me to put more foxes, or talking animals in silly hats, into the narrative, but they'll help me avoid the story turning into a stew of "things Seanan wants to play with."
What are your narrative kinks? How do you feel about their use, and how do you react when they get overused? What narrative toys would you rather never came off the shelf again? Enlighten me!