Fan fiction is mind poison to me, for a variety of reasons. The most glaringly obvious flaw is that most people who write fanfics are terrible writers, so it wouldn't even matter what it was about, it's not going to be any good because the author is a hack.
But then you move onto the actual subject matter, and the typical scope of fan fiction ensures it doesn't even matter if the writer can produce a decent turn of phrase - it's going to be some wretched author-insertion fan wank, horribly nonsensical "people who should not ever get pregnant/have sex with each other suddenly do!" scenarios, or a ridiculous cross-over that does not make any sense. Plus it feels extremely lazy to use established characters to populate your twisted fantasies spewed out onto the web, especially when you're probably making them say/do things they never would in the official version of whatever you're making fanfic about.
And on the rare occasion where a fan-fiction author produces a work that's actually decent and respectful of the source material, I still have a personal mental block stopping me from enjoying it, as I have this hang-up about stories being canonical: I don't like reading things that verge from canon, even if it does so in an entertaining and competent way. Heck, I hate it when established canon is thrown out the window in official reboots.
So while I do not contend there is no such thing as good fan fiction, I am incapable of enjoying those isolated points of light in the writhing tentacle-rape filled sea of darkness.