Hmm. Interesting reaction, because when I really enjoy a world and think it's done well, I tend not to want more in it by anyone other than the original writer; because the better and more complex a fictional world is, the less likely any other writer is to Get It well enough for their attempts to follow through not to disappoint. (I base this on a number of tribute anthologies to things like Sandman and Elric and the War of the Worlds, each of which, that I recall, had one or two good stories among an avalanche of unsatisfying ones.)
I suppose that could be an argument for good writers fanficcing unsatisfying universes to make them better, but I think I'd rather have original stuff from the good writers in question.
I disagree that fans do not "get it" well enough to write a fanfic if its too complex. Many of them are obsessive students of the story and thereby know it backwards and forwards. Sometimes the creator purposefully leaves an event/situation or character and/or his behavior open-ended for further contemplation (especially, but not exclusive, to episodic TV), plus fans desire to extrapolate on what could have happened before, in between or after x, y or z. All the answers are not there. And I have yet to find many stories that completely satisfied me - and I'm surely not the only one who feels this way - not to mention the others who just like the challenge of creating an addition, like filling in the blanks, to a story they really enjoyed, not necessarily to make it better, per se. Good writers are not immune to making bad choices, usually messing up a good thing when their egos begin to inflate and they believe everything they write is golden. Oh, contraire.
Fanfic, to me, is quite the compliment to a writer, that the author's story is so well thought of that people want more. It doesn't mean the fanfic writer didn't like the story and wrote his own story to subvert the original. Quite the opposite in most cases.
I have run into a few people who don't read fanfic because they don't want to get "confused" over what's canon and what is not...which seems rather bizaare to me, as these people are very well-versed in the stories, and I would think they are highly aware of what actually took place in the original work. It's their perogative, but a rather flimsy excuse.