This is actually a post that's been building for awhile (title and everything!), but two recent events slammed into each other to make it a reality rather than a "you should probably maybe kinda do this at some point, you know, if it's not going to be too much trouble." The first was the dismissing of Amanda Hocking's work as "basically Twilight fanfic", and the second was a Twitter acquaintance of mine fretting over the fact that she has completed over 200,000 words of writing in a single universe, but it's only fanfic and thus people are (rightly) going to make fun of it. So, once again, here we go.
I wrote fanfic, or fanfiction, or transformative works, or whatever the hell else label you want to slap onto running gleefully through the wilds of someone else's creation for personal satisfaction rather than profit for more than ten years. I didn't stop doing it because I realized the error of my wicked ways and am now buckling down to suffer artistically in a garret somewhere (I'm actually writing many of the same tropes as erotica under a pen name and discovering that people aren't lying when they say that the internet is for porn; people want to talk about sex, bay-bee, they want to talk about you-and-me…), I've stopped because I flat don't have the time any longer. I don't think that there is anything wrong with writing fanfiction, or that you have to choose between one or the other in order to be a "real" writer outside of the tyranny of spare time. (At least one prolific writer of fanfiction enjoys regular comfortable positioning on the New York Times' Bestseller List, and a handful more still get published regularly while continuing to use fanfiction as a way of blowing off steam.) It's a great way to develop craft-related muscles if you want to write original fiction, and a wide and welcoming avenue towards a fun and quirky community and great friendships if you don't. Frankly, I'm just tired of "fanfiction" being used as pejorative with a built-in escape hatch. That being said, there are still certain skills that you can learn from the writing of fanfiction, and then there are others that it definitely allows to atrophy. Let's just get the bad out the way first, mmmkay?
All of your skills at building new worlds—you know, those things that make readers believe that vampires can sparkle and dragons have some kind of influence of sexual orientation—are going to wither and die. Don’t freak out right away, they can be rebuilt. But the what-if impulse in other people's worlds that makes fanfiction so cool makes you absolutely crap at creating those worlds yourself. I'm not decrying fanfiction itself, but that's a skill that you're going to have to relearn. I certainly did. Luckily, fanfiction also teaches you a great deal about the art of dealing with criticism when you inevitably screw it up the first few times. (That is, if you choose to learn. I mean, someone has to keep Fandom_Wank going so that the popcorn farmers can make a living. You could be doing a public service.)
A built-in audience can lead to an overestimation of your own abilities and a consequent swelled head if you aren't very, very careful. Maybe you're the only person out there writing Nikita/Harry Potter crossover AUs set against the backdrop of the French Revolution, and these works really hit the buttons of a select few. That doesn't mean that you're ready for publishing yet, self- or otherwise. (Of course, if you can make the tragic romance of Alexandra Udinov and Blaise Zabini as they struggle to avoid Robespierre's men work, then you clearly have talent to which the rest of us can only aspire.) It doesn't mean that you're not, either, but bear in mind that fandom works off of a gift culture that encourages people to be very, very happy about works that they would have sent back for another round or two with a beta reader if they were published as original fiction.
You will become better at distinguishing between individual characters' internal voices without a word of dialogue being spoken better and faster than you ever would have thought possible.
Snagging your reader so hard that can't dream of turning away within a page, two pages if you're feeling frisky. My very first beta reader gave me the best piece of writing advice I've ever received (that is, outside of "Just take my advice, damn it", but that's fairly context-dependent): get them fast, or don't get them at all. All that someone has to do if a piece of fanfiction doesn't agree with them is hit the back-button. Most ebook sellers offer sample chapters before you commit to buying, and even in the pre-Kindle days I knew a lot of people who read the first chapter of paper books in the bookstore before they made a decision and went to the register. I've lost contact with that beta reader over the years, and I regret it, because I don’t think that I ever adequately told her how much she taught me.
Learning to work with editors and accept criticism gracefully. As the existence of 4Chan more than adequately attests, the internet has a special gift at turning otherwise pleasant, reasonable people into that old woman on The Simpsons who throws cats at people and frequently appears to be chewing on her own face. You will get bad reviews. You will get people who think that the high crime of writing a book or blog post they didn't like is grounds for calling into question your intelligence, family lineage, and species. There is no good way to respond to a bad review. I have a bit of a temper, and it took me a several years before I learned how to walk away from the computer and do my ranting in offline spaces where my words wouldn't be preserved forever.
Fanfiction is just another form of storytelling, and it doesn't deserve any more or less scorn than the novel did during the eighteenth century. Get down with your bad selves and don't let anyone make you feel ashamed if it's your preferred writing form. If you want to move into original fiction someday, take heart in the skills that you're building, because they'll serve your forever.