My point is that hacker news is to share knowledge or ideas and if you are going to say that you don't like a post then you should share with the community your point of view so we can learn from each other.
This is not the third grade, and our discussions are not chewing gum or a note being passed. If someone has something to add, they will. This is already possible. Everyone can comment on any comment.
You are asking to formalize something that already exists, and to do so in a stupidly paternalistic manner. Personally, I don't come to HN to provide feedback on every dumbass post that I think is dumb. (Though I very, very, rarely downvote. I'm far more likely to call something stupid in a comment and explain why...but I don't think anyone should be forced to explain why they think something is stupid. A downvote is a much more concise and efficient way to say it. I just happen to be a blowhard that enjoys the sound of my own voice, and so I comment a lot more than I vote.)
And, I agree with the above: Meta-discussion about karma and voting is just annoying. Stop doing it. It's irritating, and this kind of suggestion comes up every couple of weeks or so and gets shouted down because most of us disagree pretty strongly with it...just as this one has been.