I don't like fanfiction. It's lazy, unimaginative, and ludicrously self-indulgent.
Sometimes "lazy" has value. I won't defend the other two, but not every piece of writing you ever write needs to be a masterpiece. That's why I don't consider writing in shared worlds like D&D, Warhammer and Warcraft to really be in the same boat as other fanfiction, especially when those stories are encouraged by the makers of those worlds.