A pet peeve of mine is when there is, in historical fiction in general, a high degree of deliberately-archaic language in the dialogue / conversation. Just a personal "kink" of mine: I respect those who take the opposite view, and who like the author to have researched well as regards speech, and are irked by speech-anachronisms. I just happen to find the "archaic mode" jarring and distracting -- I'm happier with the characters speaking modern English, even though it's not what they would have sounded like at the time.
... I picked up a copy of a new book, a historical novel, I believe. When I got home I picked it up to try it. It opened in a late nineteenth-century ballroom where a man was flirting with a woman and she responded with a trendy phrase that didn't exist until about the time of the writing. After my eyes returned to their normal position I tossed the book in the recycle bin.
Now that very probably wouldn't have bothered me -- I'd have been happier with it than with "Oh, hoity-toity, sir !" or the like.
I've never enjoyed Georgette Heyer much, because of the characters' routinely conversing in Regency slang and idiom: the author has no doubt done meticulous research to get all this right for the period, and kudos to her for it -- unfortunately, it grates on me. And, when first hearing about "Jonathan Strange & Mr. Norrell", I thought the book would be right up my street. In fact, I couldn't get very far through it, because of its being all in "Jane-Austen-speak" (and orthography): not only the dialogue; but all the text -- with its being supposedly a document from 200-odd years ago. Indisputably, very clever on the author's part -- but it drove me nuts.
As said, just my individual "thing"; and an instance of how it's impossible for the poor author to please everybody !