I'm glad this topic came up on Long and Short Reviews, because it's one I've mentioned a few times in interviews. Whenever I'm asked how a person might learn to write, I tell them one of my best teaching tools was fanfiction. Specifically slash fanfiction.
It might not sound like a great tool - fanfiction is, by its nature, amateur, and so some of the stories posted online aren't great quality. However that didn't matter - I found them very useful. It was good to be able to read a story and analyse why I liked it or didn't like it, or how I enjoyed it but couldn't help noticing its flaws.
This one is so poetically written I gave up halfway through.
This one is enjoyable, but uses the word "release" in nearly every sentence.
This one is obsessed with food and stops the action every chapter to rave about what the characters are eating.
As a result, when I started writing for publication I was aware of a lot of the pitfalls already, although I admit that some of them (like overusing one particular word) are still easy to fall into.
I haven't read any fanfiction for a while, but I may have to have a look - I'm sure there are lots of new ones out by now.
So how do you feel about fanfiction?