You say:
I frequently see contra-mainstream comments get 6 or 7 karma;
The OP says:
arguments for contrary positions are received well and achieve high karma scores when they are of very high quality
...and yet you frame this as not sharing the OP's impression, which confuses me, because it sure does sound like your impressions are compatible.
I think I misread this:
moderately high quality mainstream comments achieve good positive karma while similar quality contra-mainstream comments stay at 0 to 2.
as
high quality mainstream comments achieve good positive karma while similar quality contra-mainstream comments stay at 0 to 2.
And, of course, the borderline between high quality and moderately high quality is very subjective. So, our impressions are mostly compatible but they differ enough that I don't see a problem.
My impression is that critiques of lesswrong mainstream positions and arguments for contrary positions are received well and achieve high karma scores when they are of very high quality. Similarly posts and comments that take lesswrong mainstream positions will still be voted down if they are of very low quality. But in between there seems to be a gulf: Moderately low quality mainstream comments will stay at 0 to -1 karma while contra-mainstream comments of (apparently) similar quality score solidly negative karma, moderately high quality mainstream comments achieve good positive karma while similar quality contra-mainstream comments stay at 0 to 2.
Do you share my impression? And if this is the case, should we try to do something about it?