My pet peeve: When the entire book is about making you loathe a particular character, who's built up more and more as the most ridiculously awful, horrid, gratuitously mean person alive, and then at the climax, the protagonist finally has the chance to get revenge, or at the very least tell the person how unbelievably terrible they are...
And then right as the protagonist is about to FINALLY unleash some karma, they look into the villain's eyes and they realize that the villain only acts so awful because they're scared and pathetic, and telling them off won't really make the protagonist feel any better, and really the protagonist just feels sorry for them. So she just lets them go without so much as a single expletive. Often without the villain even realizing that they were in trouble.
I hate this so much. Real life (as these forums can attest) is full of having to swallow your irritation and accept that awful people are going to be awful and there's not much you can do about it. But if I've let my silly beach book spend 200 pages manipulating me into hating a character with all of my heart, I want it to end with some kind of satisfying consequences for them. Not a moral lecture about how it's better to just forgive someone who spends their life going out of their way to cause pain and suffering to others for no reason, because, you know, maybe their marriage is having some problems or something. And if you would enjoy getting revenge on that person, well, that would make you just as terrible as they are! Exactly equally terrible.