Wouldn't a downvote-only mechanism make it easier to hide dissenting opinions without giving chance to show support for a position/comment? Besides that, I wonder how would the top-rated comment system work, given that posts start with 0 downvotes.
Basically, I don't think that a non-exploitable method exists, however pessimistic that sounds.
Welcome to Slashdot circa, I dunno, ancient times. I still wonder why no one has bothered cloning their moderation system - in my mind it's the best one I've ever seen on a major website.
An extra perk is that upvotes can be categorized (and downvotes) that add context. On Reddit all too often the top comment in a thread is a quick and easy joke, and the real insight gets buried. On Slashdot these two types of posts would coexist and be flagged as such.
Indeed. It's surprising to me that no-one has yet been successful combining Slashdot's idea of meaningful moderations with the almost universal practice on newer forums of promoting highly moderated material to earlier in the discussion but having the advantage decay over time if that material doesn't continue to receive strong endorsements. If demoting material had to come with a stated reason, like giving incorrect information or being off-topic, it might also mean fewer people use the down arrow as a substitute for "I disagree", which IMHO would be no bad thing on sites like Reddit and HN.
Basically, I don't think that a non-exploitable method exists, however pessimistic that sounds.