I'm not too fond of characters acting like Dr. Jekyll and Mr. Hyde (unless of course I was reading that novel by Robert Louis Stevenson) just because it makes the plot work. Sweet on this page, a harpy on the next page, back to sweet.....hot/cold/hot/cold....no thanks, unless you can give me some logical juicy motivation for the mood swings.
Not exactly a peeve, but if the author gets too formulaic, I'll eventually give up on their books or series and move on. (Unless of course the formula is one I dearly love!) But for example, there was a terrific suspense author years ago whose heroines were always named Mary (and this wasn't a series about a heroine named Mary) and whose heroes began uttering pretty much the same dialog from book to book as they moved through scenes I swore I'd read before. OK, I'm not buying the next one!
Oh, and closely related to that one, I have next to NO patience for heroines who for YEARS hide the child that they conceived with the one great love of their life and then somehow Fate brings them all back together again. It just makes me too sad/mad to think of all the years the father and the child missed out on, based on whatever whim the heroine had about not letting the guy know. Obviously my peeve occurs when the hero is a standup good guy and she could have easily told him. He wasn't some evil dude, an abuser or marooned on Mars on a covert black op. I get that the authors supply the mothers with some justification for why they kept the secret but I usually can't buy it and I'm just ticked off. So I avoid those as I'm not the target audience.
(And although there is a baby in Magic of the Nile, the mother tries to notify the father as soon as she's aware she's pregnant but there were a lot of communication challenges in 1550 BCE! SPOILER: And he meets his son at about six months old, not years later...)
OOH, can I squeeze in one more??? Springing a cliffhanger ending on me! I LOATHE those.
OK, rant over, have a nice day!