If there’s one thing I’ve done a lot of over the years, it’s critiquing. I have come to the conclusion that most of the time when someone does a poor job of it, they simply don’t know how to go about it.
In my experience, the most valuable thing a critiquer can do is simply write down how they react to the story as they are going along. Something confusing comes up? Say so. Don’t like the hero? Tell the author. If you can also say why, that’s great. But the absolute best thing a critiquer can do is simply allow the author to watch them read.
There have been many times I would dig in my heels and not fix something when a critiquer went to a lot of effort of telling me what to write in place of what I wrote. The same thing in the hands of a critiquer who simply says “I don’t like this part. The hero looks like a dweeb” will get an immediate revision.
Probably the most annoying of critiques is when the critiquer goes through meticulously converting your voice into hers. I don’t want every word altered to suit the critiquer’s taste. Tell me when a phrase doesn’t work at all, or simply leave it alone. This tends to be done more by novice writers than those with more experience.
As to those who viciously tear into someone’s work for the thrill of being able to hurt someone, I tend to blow them off. Their power trip is blinding them so much I seriously doubt they can actually see what I wrote. Why should I care what they think? Besides, those people don’t generally last very long. No one wants to hear what they say, let alone bother reading their precious work. In a small group, they simply aren’t invited back. In a big one they are either reviled, or ignored.