My reading pet peeve is when characters make a complete 180 in their personality during the book.
Most of the time, I finish the book and give it a poor review. However, in the case of Heinlein's Friday, I threw it and the rest of my Heinlein novels in the trash and haven't read anything of his since. I realize the man grow in a different era with different values and his books all have a tinge of sexism to them, but what he did to Friday and the smarmy way he did it outraged and disgusted the feminist in me.
In a nutshell, strong and smart female character spends the whole book fighting to become recognized for her skills and to reach her goals, finally makes it as a "Person of Importance" and then decides she's happiest as a housewife barefoot and pregnant. What a huge letdown.