Site admins should "buy upvotes" a few times, at different hours of the day, and aggressively ban every account that's a repeat offender. But, really, upvotes are always easily exploitable and often lead to a sort of herd mentality among commenters. This is why Reddit is the definition of "echo chamber." HN is much better, but it would still be improved if upvotes and downvotes were done away with entirely.
1. "shadow moderation" (Robert Hawkin's term, see https://www.reveddit.com/#spread_word)
2. "The masses" entered. When the site was smaller, diversity was expected (as well as the average technical proficiency of the users). With larger numbers, diverse thought held by minorities is harder to hear.