I don't remember ever being a fan of, er, fan fiction. I do write the occasional Legend of the Five Rings story every now and then, but I think that's about it.
That said, I have nothing against well-developed universes. In fact, I like well-developed universes. It's easy to admire the construction of an all-encompassing setting and marvel at how the little details fit together so well. But I have to say that it's also just too easy to set up camp in such a playground and start fooling around with the furnishings.
Fan fiction is actually a bit of an exercise; It tests how well one can incorporate his or her own stories into a world that someone else created, and it pits those stories against a loyal fan base that finds it easy enough to tear the original world-tales to shreds. Experienced writers should note that there are very few pieces of fan fiction that gain the respect of a significant portion of the crowd.
What fan fiction does is impose a limit to a writer's imaginations, something that standard writing doesn't necessarily do. Fan fiction literally tells you that you can write whatever you want, provided that you stay (reasonably) true to the logic or physics of your chosen universe. Snow White must associate with seven dwarves. Kirk, Spock and McCoy must be the top three officers of the original Enterprise. Every Gundam series must have a mysterious pilot who wears a mask...
You might note that the three examples I've given above are perfectly breakable: Any fanfic writer can come up with an eighth dwarf for Snow White, or supplant McCoy's place in the Enterprise's hierarchy, or produce an entire family of masked Gundam pilots. But this is precisely my point: Despite the change, Snow White still associates with dwarves, the Enterprise still focuses on its officers, and the mysterious Gundam pilots still wear masks. These are fan fiction's limits at work.
A lot of writers of fan fiction find their imaginations impaired when they delve into general writing. This is not much of a surprise.
Conversely, a lot of general writers find their works stunted when they delve into fan fiction. This is also not much of a surprise.
The tricky part, you see, is balancing the two different mindsets. One can't simply turn the limit settings on imagination on and off like a light switch. You start out being either one or the other, and it takes a hefty amount of effort to transcend the two realms.
I believe that fan fiction is good exercise for writing in general. Putting together a story under a given established background is tantamount to being given a specific subject for your next English assignment: It forces you to write something that you might never otherwise have written, to consider things that you might never otherwise have taken into account, and to possibly even figure out something that you might never otherwise have learned. Fan fiction may or may not be more difficult than fiction in general, but it is the transition between the two that can potentially make us into better writers.
With that in mind, I would also like to see more fanfic writers delve into general fiction. I don't believe that it's viable to revolve one's stories around the same established universes forever. In fact, I think it's unhealthy - it indicates an inability to let go of a creation that may have already gone past its prime. General fiction, with its encouragement towards stretching the imagination, would obviously provide a good exercise in this case.
With regards to fan fiction, I suspect that what I'm really going for is some form of interplay between the two. Both deal with different, perhaps even fundamental approaches to fiction... and if there's anything that I feel we need, it's the acceptance and use of various approaches towards the way we write.