Yeah, it would be needlessly noisy in public form. However, it can be helpful for people to get feedback directly.

Would a private reply that only the submitter can see be valuable? It would help the community give direct feedback to the submitter or commenter without adding noise.

Seriously, are you really taking your posts to HN as so important than they need to receive review from peers? It's a forum. It is informal, it is not a vetted peer-reviewed journal. If someone wants to reply, they will, and it should (generally) be public (and if it shouldn't, the responder will look up your email address and send you a message, assuming that's possible)...if they don't, it's dumb to try to force them to (privately or otherwise).

I was not thinking in karma, actually I don't care about it, at least not now but 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. (btw: sorry for repeat this post)

I didn't mention karma in the comment you're replying to. I understand your suggestion, and I disagree with it very strongly, and I said so and explained why.