My most common:
Blood
Vore
Death
Blood doesn't always mean vampires and it doesn't always mean there has to be a lot of it. Sometimes a little can go a long way depending on my writing partner. I can write with either extreme and people tend to think that it means there has to be a zillion slashes everywhere when this isn't necessarily the case even if it's something I'm willing to write/explore. I think the biggest thing is that with blood people automatically turn to vampires. Maybe this is out of fear of exploring blood as a concept within an RP? Perhaps. But, I don't think this is always a necessary factor and, to be honest, it can be quite tiresome when people constantly respond to an RP requesting blood with some vampire trope instead of something else that might involve knives or something even more unique.
Vore is something that people also tend to have misperceptions over. Now, while it means eating someone...it doesn't necessarily mean you have to eat them entirely (though it can, it depends on how extreme you want to go). But it can be quite erotic if you attack it from the angle that you're trying to get your character as close to your opposite character as possible, so one quite literally takes a bite out of the other to gain that closeness. Of course, this can link to the above blood idea. A great example of vore, at least in my own opinion, is in the Anime 'Rin: Daughters of Mnemosyne'. There's a scene where Rin starts to get consumed by an Angel, but stops it by killing him. But both are quite willing and consensual about the process before she snaps back to reality and finally stops him. See, their bodies are drawn together and desperate for the other. So, the Angel wishes to consume hers in order to achieve total closeness and she wants that closeness and thus doesn't mind being consumed while they have sex in the process.
Death. Honestly, people cringe at this one like I've never seen. I can't even begin to describe how many people get bothered when you mention the 'D' word. Suddenly, you get angry people or people trying to persuade you to go a different way with your ideas. But really, the way I see it, death is quite natural. It happens to everyone, so why not your characters? Besides, there's heaps of way you can tackle this to make everyone happy as well. You can bring in concepts of regeneration depending on the story, you can use it as a means to bring in more characters or as a plot device to kill off extraneous characters or even prominent characters, sometimes it just makes more impact to kill off a main character towards the end of a story to pack that final punch bring emotions to the forefront for the readers. Death doesn't always have to be about violence--though it certainly can be and I'm hardly opposed to writing it as such--but it can be about several things. The key is that it makes sense with the story, the natural progression of how the characters grow and develop and the way in which such an event might impact everyone involved in the process of the story. An added note here....the good thing about the fictional world is that our characters are indeed fictional. We can always bring them back for another story if we're really that die-hard or we always create more characters for our stories to fill the void and isn't it wonderful that we can create lots of characters?
Anyway, those are probably my most difficult writing fetishes.