My guess is that the justification for requiring a certain amount of karma to downvote is that if you have that, you've given evidence that you understand the t

My guess is that the justification for requiring a certain amount of karma to downvote is that if you have that, you've given evidence that you understand the tone and rules of the community enough to be trusted to help moderate it. This assumes that people upvote comments not because they agree with them, but because they contribute to the kinds of discussions HN wants to have. In practice, of course, that's not what gets upvotes a lot of the time, so I'm not sure that particular "game mechanic" is working as intended. To be fair, there is no practical way I know of to accomplish that intended goal, and this is as good a solution as any.