Well, let's get my kinky likes out of the way first: I'm nuts about tentacle porn, plants with a mind of their own, and the idea of sexual submission to machines. Boy am I embarrassed to admit all
that in public!
As to the less kinky stuff, there's so much I like, it's easier to list what I don't care for, which would be furries (by that I mean ears and tails, I'm like WTF? about that sort of thing), violent rape, and anything where males are drawn to look like females, along with anything genderbending or crossdressing. Also, although I do like manly men, I really don't care for yaoi of ridiculously over-muscled guys. On the other hand, I'm a sucker for really good drawing, and an individual work involving any of these I might like if the art is particularly good, or if it's done for comical effect.
Also, there are any number of exceptions to this list. I'm not fond of "ears and tails" humanoids, but aliens, monsters, supernatural beings (our friend Lucifer's avatar, for example), and bestial creatures (werewolves, for example) doing it with humans is perfectly OK by me. So go figure.
Apart from these few things (which now that I think about it, probably covers at least half of the yaoi I've seen

), I pretty much like it all--just about anything involving nudity, erections, and climaxes. (And big bazooms, but that's a different subject altogether

) I don't quite understand the apparent Japanese obsession with copious body fluids of all kinds, but hey it's just part of the game.
Probably my least favorite thing of all is what comes to mind when I hear the word "yaoi", that is, a drawing of two skinny guys, fully clothed, smooching. I'm very much the un-romantic type, and my idea of something cute, sweet, and sensitive is two guys looking lovingly into each others eyes and masturbating together.
I also have a thing for seeing guys doing it in public, or in places where people don't usually have sex.
As to slash and This-x-That, I get a kick out of seeing guys from cartoons I recognize doing the wild thing, but my experience is limited, and three quarters of characters I don't know who they are. So apart from Last Airbender, DBZ, FMA, NGE, and a few others (Pee-kah-CHOOOOOO!), the identity of the actual characters used is neither here nor there to me. Reading through the comments, I feel like everyone knows these characters SO much better than I do, and that I would get so much more out of the art if I knew more.