I once wrote a short fanfiction, though it was quit a bit different from many any other fanfic I have ever read. Admittedly, I read very few of them, though. Honestly, on a theoretical level, I would love to support fan fiction. Think about this: literary arts education funding is being cut drastically all around America, and it is difficult for young writers to find outlets for their craft. Fan Fiction could -ideally- serve as a safe place for young people to find their footing as artists, surrounded by other people who share the same passion as them.
However, this is not the case with most of the fan fiction I have read. We have all heard the stories of some spectacularly bad works of fanfiction, and I am sure that there is so need for me to repeat them here, but one thing remains true, generally: fan works are often terribly self-indulgent and inept. That is, they are often written to please the author's most base and crass fantasies, and not as homages to the themes, ideas, and artistry of the original works. Certainly, matching the sheer craft of one of your favorite works is difficult, and no one should feel bad for not living up to those ridiculously high standards, but some standards must be kept, if only for the sake of not wasting the audience's time.
The fan fiction I wrote did not have an action sequence, nor did it have a sex scene. In a sense, I did write it to fulfill my own fantasies, but those were not so obvious or prurient. I wanted to bring some older characters back, and allow them to have a conversation with my own original character, just so I could grasp how their arc as characters informed them, and how they would choose to communicate what they had learned in the original work to my character. I realize that this may be seen as something of a conservative approach to fan fiction, but I think one that -by extension- shows my objections to many works.
On the other hand, someone else did write a Goku and Anne Frank fan fiction. . . which on one hand is idiotic, and the other, possibly brilliant:
http://www.fightbait.com/time-anne-frank-goku-fanfic/