Agreed, the karma barrier does help a decent amount. But even on HackerNews I see the same "downvoting mob mentality" occasionally where all comments expressing certain views or disagreeing with comments of more popular users are reduced in visibility because of down voting. In a way, it gives the higher ranked users a platform to push their own posts higher/other posts lower, not unlike the way capitalism can widen economic gaps between the rich and the poor.

While it is helpful to hide comments that don't have much to do with the discussion, upvoting related comments has the same effect without the downside of making the comment invisible to people who want to see it and without impacting the user's status on HN.