I would simply say a lot of contrarian viewpoints, regardless of merit, are downvoted into oblivion. Once you get hammered on something you simply had a different viewpoint on? Users tend to stop commenting for fear of reprisals. Your "karma" on here is not easy to obtain so when people downvote you simply for a differing opinion, it makes it less likely they will wade into a discussion again and find themselves on the wrong side of some Hacker News diva having a bad day.

This means you have a lot of people (like myself) simply opting out of a lot of discussions because if you're not on the right side, your comment will get downvoted immediately. There is no data point on people like myself, so there's no way to tell people that the quality on HN is declining, everything is just fine and normal when in reality, you have a lot of users who aren't engaging for fear of getting downvoted into oblivion.