The problem is that this produces an echo chamber, the things that majority 'agrees' with hits the top, and the things that folks 'disagree' with sink out of site. Upvoting/Downvoting comments shouldnt really be based on 'aggreement' but instead based on 'interesting', upvoting interesting or thought provoking comments regardless of wether you agree or not leads to more interesting conversation...
Using upvotes freely but downvotes sparingly produces something else (which I cannot mention by name without this comment going 'dead'). The willingness to disagree is a large part of what makes HN what it is, and I think that downvotes help foster the sort of atmosphere in which that is possible.
Really, contrarian viewpoints are hardly an endangered animal here.
