Characters are really importants, most people read a FF because of the couple its based on. I'm not saying thats all fair, but I guess I can't deny the fact that the popularity of a couple adds to the FF. [ex KaSh/AR vs. Sashank-Padma/Shubh-Keerti, most people would be more excited for the KaSh/AR fic..]
Concept is a major attribute, sometimes, when I go to read a FF, and they have concept thats too familiar, or used to much..I usually don't read it, because it'll feel like you're reading the same thing over and over. [ex Good girl meets bad boy come good boy, so used, so old!]
Whats included in the story, I'd rather read stuff thats casual, not to dramatic. Something I can relate to or something I've seen or experienced. [ex having the main character wanting to sacrifice any and everthing for her/his family vs. having her/him love the family, but thinking about themselves twice first]
This is all strictly from a readers point of view!
Krupa