The reason HN doesn't need downvotes is that HN, unlike Reddit, kills lame articles. On Reddit, users need downvotes as a way of saying an article is lame. Downvoting is the only way you can get a (nonspam) submission off the frontpage. But on HN you can flag it and if it's bad the editors will kill it.
We can thus safely assume a nonlame set of articles, and we also (so far at least) assume nonlame voters. And if you only have nonlame voters voting on nonlame articles, upvotes should be enough to pick the winners.
replies(5):