That is, it's often considered OK to say things, as long as they are properly hedged and constructed, that if said more vulgarly or directly would be condemned for violating wheaton's law, and which in other communities would be condemned for a variety of reasons.

Yes, and this is best, is it not? I enjoy reading what people have to say, even if their views are directly in contradiction to mine. I've changed my views more than once because it was correctly pointed out to me why my views were wrong. http://wiki.lesswrong.com/wiki/How_To_Actually_Change_Your_Mind

And about being vulgar, it's just a matter of human psychology. People in general - even on LW - are more receptive to arguments that are phrased politely and intelligently. We'd all like to think that we are immune to this, but we are not.