I was trying to remain agnostic with regard to any specific norms. I'm not worried about particular values so much as the possibility of differentially discouraging sincere, well-informed dissent in newcomers relative to various forms of insincere or naive dissent: over time I'd expect that effect to isolate group opinion in ways which aren't necessarily good for our collective sanity. This seems related to Eliezer's evaporative cooling idea, except that it's happening on recruitment -- perhaps a semipermeable membrane would be a good analogy.