Feedback should be positive only

+/- feedback schemes work when it's likely that there's a common scheme behind the ratings. For example Wowhead has a comment thread attached to every item, skill, creature, zone and so on in the game World of Warcraft. People give positive ratings to comments which helped ("check the cottage, sometimes the Blacksmith is there", "the equivalent Alliance quest is <link>") and negative to those which didn't help ("the Blacksmith is only there at night" (no he isn't), "this game sucks" (so what?), and so on). The system automatically hides comments with a negative overall score.

Or consider Stack Overflow, the answers supplied can be downrated, an answer which is wrong, or unhelpful gets a bad rating. But comments attached to answers can't be downrated, only positive feedback is allowed for those.

Slashdot is actually a very interesting example because of meta-moderation. In my experience this is actually pretty effective, but it requires a vast community because you need to be able to rely on a randomly chosen participant being unlikely to know any of the people whose comments they're meta-moderating. LWN is too small a community to try that.