Almost always, romance can't be the main plot. Subplot at best. It has to be a hell of a fanfic to get me with romance. I'm just so extremely picky with romance, I'm the same with books. Usually something say suspense, horror, sci-fi...or drama about the guys and their relationship as a group. Basically the genres that never pop up sadly. Has to be Brian, Nick, or Group centered to catch my eye.
Characters need to have depth, I am SO anal about characters when I write (ask Julie lmao), that I'm the same way when I read.
Female characters are okay, HOWEVER, it can't be all about her. She needs to have purpose in the story, in relation back to the Boys. She needs to have flaws and be real and easy to relate to. A good example of good use of a female character? Claire in Broken.