What makes a good fan-fic?

[Human No More]

Originally Posted by Clarke

Though AFAIK, the reason the majority of fanfic is crap has nothing to do with the fact that it's fanfic, but because 90% of all writing is crap. It doesn't fail at being fanfic, it fails at telling a story, whether because the characters are wooden, the writing is structured badly, the plot is nonsensical, or whatever. (Or, even more annoyingly, the author can't spell or compose a sentnece/paragraph properly.) In original fiction, 95% of that is never published, and so we don't see it.

Yep. That's the main difference. Even well-written fanfic would often (but not always) still only be a mediocre to poor original book. It's also easier to write with characters, worlds, universe constraints and continuity that has already been established then creating original ones (even if some authors do ignore these in some of the lower-quality fanfic). There are exceptions, of course, to fanfic being low-quality.

IMO, none of the above mean that fanfic can't be different, very long, or even completely bonkers, it just needs to carry over the "style" of the original. Harry Potter And the Methods of Rationality, for instance, starts out as very good fanfic, because it keeps the feel of the series, but shows how it's changed when Rational!Harry tries to mess with it. (I've not read to the end, but it apparently it becomes worse in that sense because it starts focusing on game theory and I-know-you-know-style exchanges, rather than the conventional elements of HP)

Yeah, that's one of the few fanfics I've tried to read, and I gave up on it for that reason (and apparently be becomes an uber-Mary Sue as it goes on too).

With Avatar, there's the example of Between Worlds. It's a nice fanfic, has some very well written scenes... and has absolutely no understanding of the background or technical details of Avatar, to the point that its whole premise is completely invalid (and then it jumps genre, leaving "appropriateness" behind with it), and gets so much wrong that I stopped reading it.