Do writing groups become tunnel-visioned ... and come with their own set of biases?
I believe they do, just as part of human nature. There are many studies on this regarding the outcome of focus groups, and in psychology: How 9 actors can convince, by consensus, a single test subject that his eyes are lying to him. It actually has been shown to have serious consequences in wrongful jury convictions. (By recollections of the jury that found an innocent person guilty).
You are facing the same kind of focus group. A friend of mine in advertising does filmed focus groups for radio and TV commercials, and he says there is always somebody in the group that people will start to look toward to see if they are going to speak. Sometimes a person with good or interesting observations, but usually a bit of a bully that is just expressing a strong opinion with certainty, interrupting people, making jokes about their observations, whatever.
That's human nature. Any community of people that interact a lot is going to come to a "norm" they adhere to and then lean toward it. About what is "good writing", what is a "good twist", what is "good dialogue".
You can see it now, amateur critics on line reviewing and trashing Stephen King or Dan Brown or JK Rowling for their terrible passages or scenes. Well I'd wager King and Brown and Rowling disagree, and obviously the public has decided to give them a few billion dollars for their efforts.
A camel is a horse designed by a committee. Not only that, but they are pleased with how much they've improved upon the horse.
If you want to write horses instead of camels, study horses: Best selling authors.
+1 to Mark for using such a group to become a better writing critic. I would add another step: While you do that, apply your critiquing skills to existing best selling authors, you can buy their work for 25 cents in the used bookstore. Then realize they've made millions, so if the rules you are learning about how to critique are making them look bad: It is almost certainly your rules that need to be refined or toned down, not the bestselling author. Because clearly it isn't keeping King or Brown or Rowling from producing killer entertainment, and clearly people don't care if they end some sentences with prepositions, or use vulgar American comma rules instead of the British rules, or whatever.
Stick to advice from the people you aspire to emulate. I know you cannot ask them questions directly, but if you take notes on their work as you read it, you can try to find where they have answered it by example.