cry havoc

Heh, I would definitely agree with that about Gundam Wing fanon. And, interesting, this is something that I first encountered from both sides, in the Magnificent Seven fandom. That was one of my first fandoms, that I got into solely via a friend's recs, back when the show wasn't available on DVD. About the only way to get ahold of it was to watch it on the Hallmark channel, if you got that, or to beg copies from someone else in the fandom, so a lot of the fandom had never actually seen canon. They'd just learned about everyone through fanon.

So I read a lot of fic from that same perspective, and then I actually saw canon -- and I came back to find that some of the fics I'd loved were suddenly unreadably OOC. They were still well-written, but suddenly I had mental voices for the characters in more detail than "laconic" and "Southern, verbose," and the fanon ones didn't fit, and I could see where people had taken fanon exaggerations of canon throwaway traits or lines and built on them as a solid foundation, instead of making them be a "shadow" from the canon foundation.

(Oh, and I'm here via metafandom, and I love this post! It sums up very well a difference that I think is very key. In fanfic, you can assume the reader's emotional involvement, and canon as a basic template the reader already has. In original fic, you need to start from zero on both of those. You can't ever assume that the template they see is the same as the one in your head unless you've put it on the page already.)