"Fanfic asks so little of a writer that it's almost impossible to judge how talented that writer is. The world basically supplies a pre-made setting, conflict, characters, and more often than not strongly influences the plot, too; it's the same kind of problem, albeit even more pronounced, that someone writing sword-and-sorcery fantasy has to deal with. In other words, there's so little of the story that's really yours, so the reader is only invested in the story inasmuch as they are invested in the universe.

I don't want to suggest that you shouldn't write fanfic if you enjoy doing it, but if you want to become a serious writer you have to deal with making up your own settings, plots, and characters."

-Romper Billson