Under that assumption, upvote should not be a way to voice agreement, as that implies that downvote is disagreement. But downvote leads to graying-out and disappearance, which is just perfect to induce the hive mind.

And yes, downvoting into oblivion articulate comments not at all devoid of content really has become endemic here, and many (though certainly not all) of those I read anyway were obviously downvoted for disagreement, often political. The result is a strong hive mind at HN. (I've seen seen that opinion on HN voiced outside of HN more than once.)