(1) The war between bookmark usage and marking a spot in a book by folding the corner of the page. The thought of someone folding book corners makes my teeth grind together, but others love it. This battle will never, ever end.
(2) When writers make characters do blatantly stupid things to make the plot move forward. There’s only so much obliviousness and stupidity in the world a reader can take.
(3) Ugly book covers. Who designed you?! Who looked at that image or those drawings and said, “Hmmm yes, this will sell nicely”??!! So many books end up getting ignored because they have heinous cover art. It takes effort to make things look nice, but damn, YOU COULD AT LEAST TRY.