I also disagree with the idea of karma-driven downvote system. However, I wrote the above with the assumption that those who can downvote know HN values and enforce them. As you pointed out, yes the karma-downvote thing doesn't quite accomplish it's goal (assuming the goal is to allow senior members to act as enforcers). It should change. However that wasn't quite the point of my post.
My original point is: if you gonna say smack, gimme a damn good reason why.
Maybe enforce a stealth downvote. This would be a case where the downvote button is visible but you use heuristics and probabilistic models to determine to whether actually accept the guys' vote or not, and if yes, what weight to assign it.