This isn't the only fandom in which I read fanfiction, but this is the one that seems to have the best writing.
At least our fanfiction writers use good grammar, for the most part. Even those who make errors at least try
. Some writers in Other Fandoms have such bad spelling, grammar, punctuation, etc. that it is hard to follow the story. Others will tell a good story but have such quirks (e.g. switching narrative tense at random, consistently mixing up homophones) that one has to make an effort not to give up on them.
And, thankfully, none of these seem to have shown up: http://tvtropes.org/pmwiki/pmwiki.php/Main/MarySueTropes