OT:
Fan fiction is a problem for authors because while the settings/characters and whatnot are basically under copyright by the author, a fan can create a unique story using that world and characters. It's muddy as to what the author can and should do about it. Most of the time the fan isn't generating money so there is no law/harm to the author THEORETICALLY.
There is a case oft-sited of Marion Zimmer Bradley who was writing a novel in her series, and found by reading fan fiction that a fan had taken a story in a similar direction. She then refused to publish the novel because she was afraid that there would be accusations on all sides.
http://fanlore.org/wiki/Marion_Zimme...on_Controversy
The problem with fan fiction is the fans usually read all the novels. Writers have a style and that style can become somewhat predictable especially if you've written multiple series. For example, I happen to prefer happy endings so that is what I write. I tend to write series where the main romantic relationship(s) doesn't happen all in one book, but rather develops over the entire series. So let's say I start a third series. It would not be unusual for me to do the same thing. A fan of mine, who likes that type of style, might write a short story or other story that "Got to the finish line" before I did. And they might use some of the same ideas I would use. This is essentially what supposedly happened to Zimmer-Bradley -- and she happened to read the story and thought, "Uh-oh. I"m in trouble because I don't want to deal with this person thinking I used her/his idea." It was too close to what she had planned.
Fan-fict is still a story. There are a lot of writers who see similarities to other works and assume their idea has been "Stolen." Someone sued Rowlings because the author had ALSO written a fantasy about an orphan boy and it happened to have an owl as a messenger bird (I think Potter used a hawk? I can't remember). The point is, the lady saw these similarities and was positive the Rowlings had stolen her ideas and implemented them. The judge found differently.
Fan fiction can put the author in a very difficult position. That does not mean it should be plagiarized, but when you have a bunch of fans basically writing out different character scenarios, it should not come as a surprise to anyone if more than one idea follows the same arc.
As an author I wouldn't want to read fan fiction for my characters. It's bad enough when a fan writes to me and says, "Hey, for the next book, wouldn't it be great if..." and then proceeds to lay out their ideas. Sure, an idea is about 1 percent of the work. Implementing it is 95 percent and then there's some technical formatting, etc. But fans may see that 1 percent and decide that means..."My idea, I should be paid for it."
The more popular the author, the more fan fiction and fan mail and the more ideas...