In my experience, it's fandom and amount of sex that determine the most hits. People like smut, and if it's in a popular fandom, they like it even more. :-) There are writers who used to write LOTR RPF brilliantly, but those fics have relatively few hits compared to more recent stuff they've written in newer fandoms. Abundantlyqueer comes to mind. It's not that the newer stuff is better, imo, just that LOTR RPF is no longer very popular.