For me, length is the most important factor. The only time this wasn't true for me was when I was first getting into fandom and would spend hours and hours every day, so it didn't matter how long anything was. Ever since then, length has been huge. Cuddling up with a six-hour fic is nothing like munching a 20-minute vignette or a 2-minute drabble. Now that I only occasionally dip a toe into fandom anymore, I definitely wouldn't want to get slammed with a 10-hour epic when I'm in the mood for a couple drabbles, and on the rare occasions when I do want something long, I do want to be able to find something. For me now, genre is the next most important thing: "humor" or "porn" says a lot more about what the fic is than "Harry/Ron" or "Harry/Draco" does. Any of the other factors besides those 3 are interesting but not so important.