People seem to get really confused on this issue. They think an author will get angry with them if they dare to question anything we've done. This is not the case. After all, every author has faults. All of us have a lot to learn and if we could see all the mistakes we make easily ourselves, then we wouldn't keep doing them (especially with all the writing and rewriting we're constantly doing.) Not only is there a lot to learn when it comes writing and how to communicate our stories the most effectively, but we are all tainted, to some degree, by our own emotional blinders that do not allow us to fully distance ourselves from our work.
That being said, there are some things that do frustrate authors when it comes to negative opinions.
1. Being told what's wrong with our work after we have been published. I mean, we expect people to dislike the things we've written because we're not perfect and we must sit through book reviews and reader reviews telling us all the ways in which we are imperfect. We can also learn a lot from these reviews.
But, it can also be really frustrating because we can no longer change what we've written. We might even agree with you and suddenly see a flaw in our writing, but there's nothing we can do now to fix it.
The more feedback we can get before being published, the better.
2. Being treated like we are lazy or idiots. A review of someone's work that I once found completely unacceptable (not mine, I have not yet been published) was a book reviewer ranting about all the mistakes in the novel and how they thought this must have been a first draft. He said that he wanted to quit book reviewing because he was sick of the burden of having to labor through all the stupid writing out there.
It was completely disrespectful. It's one thing to point out the spelling mistakes and plotholes and another thing entirely to get self-righteous about it.
It may be hard to believe, but most authors aren't lazy or intentionally stupid. To write a novel, you must do a lot of research and rewriting and most of us do this excessively. Yes, we still make mistakes and have a lot to learn, but by belittling the author and telling them they obviously didn't try at all or they're stupid, you're not actually helping them. You are just insulting them.
And I'm pretty sure most of these opinions come from people who have never actually tried to write a novel themselves. Even writing a first draft can take a long time and a lot of effort, so while the results may not be perfect, it's good to respect the effort and actual study/research that most authors put into their novel.
3. When uninformed people express their opinions. I wrote a story once about a girl who was raped. Another girl read the story, informed me that she had once been raped, and told me all the ways that what I wrote was wrong. I completely appreciated her doing this and it helped my writing out immensely.
Another guy, once learned that I write about female necromancers. He told me that I should have never started writing and that I had obviously not done any research because the word "necromancer" didn't exist and even if it did, it was literally impossible for females to become one.
The word necromancer is found in dictionaries. I know from my extensive research that necromancers exist in real life (although not in the way I have written them.) There are other books out there written about female necomancers, including one of my favorites, the Darkest Powers Trilogy by Kelley Armstrong, so everything he said was false. And he was trying to belittle me and make me look like a huge idiot because of it. Of course, I got frustrated.
We want as many opinions from readers as possible and we don't expect them all to be educated, but if you are going to argue and swear up and down that your opinion is right, then you should make sure that it's actually true. Because if you give an author false information and they believe you without getting angry, then you might have just ruined their story a little rather than helped them.
Overall, I just want to say that when critiquing a novel, especially an unpublished one, do not be afraid to tell the writer what you think is wrong with their work. We're not as sensitive as you believe and I hope with this guideline of things that truly do upset us, you'll understand how to give a critique most effectively.
Note: I don't believe there's anything wrong with giving book reviews. They are necessary and important, even negative ones. I'm just listing them here because they are something that can be frustrating to an author.