There are two kinds of criticism: BEFORE the book is published and AFTER the book is published.
BEFORE a book is published it goes through several rounds of editing. The first round usually deals with plots that don't quite work, characters that aren't quite believable, clumsy prose, chapters that need to be move around, ideas that need to be thought through. The next round gets into the finer points: too much description or not enough; dialogue that sounds too young to fit the character, or too old, or too formal, or too colloquial; not enough emotional content, or too much melodrama. The third round involves fine-tuning all of the above. Later, fact-checkers make sure dates are correct and there are no factual errors; copy editors check the spelling and grammar (the Grammar Dragon would be proud). At last, the book is ready to go to print. I'm used to this kind of criticism, because it's always positive and usually helpful. It goes with the job.
AFTER the book it's a different story. It used to be that teachers and librarians and regular book reviewers gave their opinions of a new book. Now everybody gets to express an opinion. This is when I get really touchy...and sometimes grouchy. I get lots of email from readers who love my books, or at least like them well enough to tell me so, but there are apparently plenty of people out there who are not so favorably impressed, and they are not fainthearted about saying so in a very public way, like on amazon.com. And it's way too late then to do anything differently.
The strange thing is that I can quote the bad reviews almost word for word, but I have a hard time remembering the good ones. I guess I need to develop a thicker skin, or not read the reviews, or just not care. What do YOU say?