There is really only one reason in practice HN comments are downvoted and/or flagged[0]: they're telling other commenters what they don't want to hear. [0] as

There is really only one reason in practice HN comments are downvoted and/or flagged[0]: they're telling other commenters what they don't want to hear. [0] as opposed to being removed by moderators, though dang has been known to use moderation as a super-downvote.