Criticism is a part of life. It's even more so a part of the writer's life. From readers to editors to agents, opinions are everywhere, and sometimes it can be hard to process them and keep from having a part of yourself crushed under all that weight. Even worse, so many times you never get to hear those opinions. They just reject your work, that's all you know. That's one reason why critique groups are invaluable. First, they help you polish your work. Second, they help you test reader reactions. Third, you can learn by critiquing and reading as well and see what other writers are doing. Fourth, networking.
However, there is a certain protocol involved and some people seem to have a hard time learning it.
1) In offering critiques, keep the sarcasm to yourself. Comments like "Oh come on, you've got to be kidding me! You expect me to buy that?" are not appropriate (actually heard that one once). They are not supportive or encouraging, and they add nothing of value to help the writer improve their work. Be honest, would you like it if you got a response like that?
2) Be careful to avoid applying your own assumptions and bias too heavily to another writer's work, especially when they are writing from a culture or perspective far different from your own. For example, I researched and wrote a short story about illegals crossing the US-Mexico border who deal with the Border Patrol and aliens. Because of my concern for accurate portrayals, I had the story read by a friend in the Border Patrol, Mexican friends, even an illegal. They helped me ensure I was fair and unbiased and got the facts right. But I had several critiquers (white) who suggested it was racist. And they also said the Border Patrol stuff was unrealistic. None live on the Border. None had any connection to Mexico or immigrants. They just read into it with their own assumptions and called me out. It was annoying, unhelpful and frustrating.
For example, I don't like stories with a lot of foul language or graphic sex of any kind. I have to read them in my secular critique group, and if I think it will limit salability, I mention that. But beyond that, I leave it be. I would never write that. In fact, I think it shows a lack of creative effort. Shock tactics have been so over done, people are numb. If you can't use your creativity to find more colorful words to tell your story than four letter ones, you're not trying hard enough. But I don't penalize writers for that opinion. I just apply it to my own work and my purchasing choices at the book store.
It's okay to say, this bothered me because... It's okay to suggest something was unclear... It's okay to say you're worried something might come off negatively which wasn't intended... But give the writer the benefit of the doubt and be helpful. Don't insult their intelligence by implying you know more when you don't know what experience or research they have to back it up.
3) Find as many good things to say as bad if at all possible. I always point out things I liked from creative descriptions or snippets of dialogue that stand out to characterizations, etc. And if I think the story has potential for development, I tell them so. If it doesn't I don't, but I want to at least leave them feeling I appreciate something about their work.
4) Critique others' work if you expect them to critique yours. Don't join the group, submit a bunch of work, then sit back and wait for people to edit it for you. Get your hands dirty and read theirs. Offer them feedback. And do it in a way you'd want someone to critique yours (per the protocols listed above).
There's probably more I could get into but I think these are the essentials, and, if followed, will enable you to have a good and productive critique group experience for all involved. After all, the purpose of critique groups is to help each other improve and grow. If it doesn't accomplish that, it's not worth investing the time.
For what it's worth...