10. "Fans of Author X will love her latest offering." Yes, because loving the author's books is the definition of a fan. Can be a cagey way of saying "this book follows the exact same formula as all of Author X's other books" or "only people blinded by love for this author could possibly enjoy this book." More often it just means "this book will appeal to those who like this kind of thing."
9. Reviews that state, often at great length, that the book did not work for the reviewer, without giving enough information for me to conclude whether it's something that would work for me. If you can't figure out why you didn't like a book, or you lack the confidence to name the book's faults, why are you writing a review?
8. Complaints that a book is "nothing but a Harlequin romance" when in fact it just has a romantic subplot. Or maybe the romance is the main plot, but it's a tragic one or dealt with in a realistic rather than escapist way. If you're allergic to romance of any kind, just say the romance was too much for you rather than claiming a book belongs to a genre that it doesn't.
7. "Anyone who wants to get lost in a great story will love this!" That describes every reader ever, and the book has never been written that everyone loves.
6. "I didn't like this because I was expecting historical fiction and it turned out to be fantasy. But if you enjoy fantasy you will love this book." One would have to be a very undiscerning reader to love everything in their preferred genre. If you don't have enough experience with a particular genre to judge whether this book is a good example of it, and you don't feel comfortable evaluating whether it's a good book regardless of genre, then don't.
5. "This book has it all: love, anger, compassion, jealousy, hatred, and forgiveness." Okay, a variety of emotions are felt in this book. And that makes it different from every other novel... how?
4. Claims that a book contains excellent writing, by people who lack the ability to judge writing style. I can almost forgive this in 5-star reviews of bad books; some people just have bad taste and I try not to judge them for it. But not in the 1- and 2-star reviews saying things like, "The writing was great, I just couldn't get into it," when the writing is so clunky you wonder if the editor was asleep at the wheel. Hint: "this books contains descriptions of things that are beautiful" is not the same as "this book is beautifully written." If you don't feel confident passing judgment on style, don't claim it's great just to say something nice.
3. "This book examines the ways we love, the secrets we keep, and the lies we tell ourselves." Come on, this is just trite and ridiculous. The book is not about "us." I am not in it, okay? I might be able to identify with some of the characters in the book... or I might not. Stop being grandiose and tell me about the characters and their stories rather than insisting they represent all of humanity.
2. The word "unlikeable," as shorthand for "I didn't like this character." Often used to describe female characters whose lives are not defined by their kindness and selflessness, or any characters who display a normal array of human flaws and weaknesses. Differences of opinion are one thing, but I often enjoy reading about these characters--and the fact that I, or anyone else, liked a character, means she is not in fact "unlikeable."
1. Unmarked spoilers. Many people visit review sites in deciding whether to read a book, and find our enjoyment diminished by knowing the major plot twists or the end in advance. Even if you personally don't mind spoilers, don't be one of those sanctimonious assholes who includes them because your way of reading is clearly better than other people's, and what kind of moron reads a book just to find out "what happens next"? Seriously, be considerate. Same goes for those who hated a book and want to spoil it for others so we won't read it: fine with a spoiler warning, asshole move otherwise.