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Metadiscussion on mailing lists almost never means "discussing the source/characters/fanworks" -- those things are simply everyday discussions. Metadiscussion means "discussing the discussion", or shifting away from the topic at hand to talk about how the topic is being discussed. This often comes about from some list members' desires to see the list stay "polite" or "nice", and avoid what they consider to be unpleasant, offensive, or "mean" language.
The term metadiscussion is not used the same way meta is used on journal-based discussion places.
For instance, a conversation may start up on a list about a new character on a show, with different people chiming in on what they think of her. Someone may say that she comes across as bitchy; someone else may respond by saying that it's better not to use such loaded terms, because it can be seen as offensive. That last response is metadiscussion, and once it starts, it can derail entire discussion threads as people begin arguing over which words are better to use, what tone is implied, who has the right to decide which words and tones are appropriate, etc.
While any topic can spark metadiscussion, it's most likely to happen during a discussion of a fanfic story, as fans with radically different approaches to public discussion of stories clash, generally along the traditional lines of "if you can't say anything nice, don't say anything at all" (where "anything nice" often means "unilateral praise") versus "once a story is posted, it's public, and people have the right to say they didn't like it as long as they're talking about the story, and not personally attacking the author".
These arguments over being nice, or not being nice, not only derail conversations but can also get incredibly vicious, and can devolve into ad hominem attacks on all sides.
In 2000, the first mailing list to expressly forbid metadiscussion started up -- Prospect-L, in Sentinel fandom. Over the next few years, several other mailing lists adopted its rules, such as Pros-Lit in Professionals.