One thing I notice reading the prompts at the kink meme is that well... I just find it hard to get into a prompt, or a fill for that matter, when Jared or Sam are put in the position of bottom or sub or slave.
Now admittedly, part of that is because I'm not attracted to Jared. Sorry, he has a nice body, but I just don't find his face all that attractive. (I may have mentioned this before*g*) But worse than that, he doesn't hold my emotional interest.
Just because I find a character unattractive won't always stop me from writing from their pov, or finding them fascinating. But even though I like Sam well enough, I just have little interest in reading stories with him as the main character. Add to that, that most people have the habit of going on about how attractive the submissive character is, which well... pulls me out of the story pretty quickly when said character is someone I'm not all that attracted to.
And really, I can buy 'Handsome' when applied to Jared, he's not my type, but I'm not blind, but when you want me to believe that he's supposed to be 'pretty', and not just the younger version of him, then you really are challenging my suspension of disbelief.
It's shallow really, but when I read a slash fic, who is on the bottom tends to depend on three things, for whom is it most in character (which should actually be the most important bit), which character am I most emotionally connected to. In supernatural that's Dean and in extension, Jensen. and third, (hides in shame) which one is smaller and prettier, which once again, Jensen
The big problem with RPS though is that there is no personalities to build on, or to judge a preference from the character on. And connection with the character depends on the individual fic and how they're written. So all there is, is looks. Meaning that lookwise, I'll always prefer Jensen as sub/bottom/slave.
Guess I know what that says about my personal preferences*g*, which is silly, because if there's women involved in the fic, I always without exception want them to be either dominant and/or on top. Which probably explains why Buffy/Spike is still my OTP, even years after the show's ended.