In general, the first question depends on the character. If I like the way the character looks, I will like the picture, regardless of who that character is. Fan art would not exist if nobody ever came up with characters anyway, and some of our local heroes are member created.
Characters like Teeva, The Bounce Chicks, Ilohnoh's Jen, Stipplewitz, Dr. soles and crew, the ladies of VU and Yenny, to me, have become as iconic as the most popular mainstream characters out there. (and that's just to name a small handful of enthusiast-created content.)
To me, it's more about the artist's mind then the picture's focus.
I know that answer wasn't really a yay or nay, I'll sum up by giving my vote to OC's, although if it's art, I'll be checking it out and enjoying it no matter what.
(I ramble...this is why I post so rarely, If I did all the time I would have no time for anything else. ;P) Like everyone though, there are certain mainstream characters I have a weakness for, so if I see a pic with one of those ladies in it, I'm very likely to be pleased.
Nudity? Yes please!
It doesn't have to have it. Especially with fan art. Some of my favorite pictures depict the victims fully clothed. In general, I really don't care either way, again, it's about the artist and his/her depiction and the spirit of the image. But this is sexually-related content. It's a fetish forum. I understand that excessive nudity can be viewed as tasteless or it can be mishandled, but as long as the artist does it with skill and class I will never complain about exposed flesh.
As far as the mood is concerned, I tend to gravitate toward themes of interrogation/torture/sexual arousal. As long as the scenario has a light-hearted feel, I'm sold. I feel it's important not to take things too seriously around here, after all, no matter the situation, tickling, at it's core, is funny.
Finally, realistic or not, an artist with a definitive style is good. I really don't even know what the difference is between realistic and hyper-stylized when it comes to art. The things one draws should look like they look in real life sure, but it's always a representation, a translation. How you choose to depict that thing is always dependent on style, whether it is "realistic" or not.
Anyways, that's it for me!