Fanfiction peeves

Did this happen to you? You discover a fanfic writer who is good, as in really good. Language, in-depth characterisation, vignettes or long stories with an actual plot, angst and humour, she can do it all. And then you read a story which doesn't feel right in a way which makes you feel vaguely dissatisfied, or even irritated. And you need time to pin-point just why, precisely because this author is so good.
( Spoilers for MANY LOVES by Jessica Walker and PARLEY by Jennifer Oksana )