I hate it when characters in historical setting fics use modern colloquialisms and slang. It's just so jarring and instantly removes me from the work. I'm willing to have some leniency here, of course, because sometimes the way people used to talk just comes across as too jarring to the modern audience and you need to adapt it a little. But a character from the 1600s who is an English noble shouldn't talk exactly like the kids I went to school with or a tumblr post.