Earlier quoted context omitted.
Maybe you should allow downvoting for everyone then. I'm more likely to downvote a dismissive reply then I am to upvote every positive reply to overtake it. And on the mention of dismissive comments: as a frequent visitor I feel emotionally safer just reading and not posting... so if there is a karma threshold for allowing downvotes, I doubt I will ever hit it.
The problem is that people would abuse the downvote button and use it as a representation of "I disagree with you" is read of "this comment adds no value to the conversation."
2- Why users with karma will have a better understanding of downvote than the passives old-regular-readers? Peoples who read HN every days love the high level of the comments here, let them have produce some feed back. Note that I just defend my interest: I'm not good at creating comment with value (so I do not comment a lot), but I feel confident to reconise good comments from bad comments.