Painfully funny. It comes with a warning, though - DON'T READ IT AT WORK! People will look at you strangely as you smirk and splutter your way through the lunch hour. -- M-A
Good fan fiction is a bizarre phenomenon, as it leaves one wondering precisely why talented people wish to waste themselves skirting around the edges of other people's universes and writing to a tiny, tiny audience rather than attempting to tell their own stories. Perhaps it's a security blanket thing.
There's also the aspect of not having the time/effort/energy to world-build for this simple story-outline that you have that already fits wonderfully into this existing world.
And perhaps also liking these existing characters enough that you'd like to see what would happen to them if X, Y, Z happened - how they'd react, etc.. --AC
How much 'world-build' does a simple story take anyway?
(world-building) Quite a lot sometimes. It's a lot easier to write stuff when the reader already understands the locations and/or the characters involved.
Personally, I would never want to write fanfcition. But it certainly does help if you can evoke a responce of "Oh, this bit is like ...." - it saves you from Tolkien?-like levels of intro.