I definitely agree. Of course writers at a fanfiction site write for fun and they shouldn't receive any criticism if the reader doesn't like their style or prefers stories more true to period, whether in content or in style. Constructive feedback is best given in private because one never knows how easily a writer might be discouraged. But it's also good that a fanfiction community has a friendly atmosphere where advice and help to find sources of information can be given, and ' efforts at being true to period is appreciated, because the general quality of writing improves that way. I definitely prefer the average quality of stories here to even some published books!
And strangely, the fact that I learned English as a foreign language makes it easier for me to spot modernisms, especially slang usage, since it isn't taught in a language course so doesn't come naturally to me.
