HN is public and anonymous and mostly manages to not suffer this problem, due to the heavy manual labour of the moderators and a whole lot of selection bias in its userbase.

To a slightly lesser extent, Slashdot has also licked that problem: for any story that attracts enough comments (more than 300) the top rated ones are often pretty good and the worthless ones start auto-collapsed. Even though Slashdot gets a lot of jip these days, some of it deserved, I've still spent many enjoyable hours reading some of the top rated comments there. Especially nice is the fact that upvotes must have an adjective attached, which gently nudges users into certain kinds of behaviour. For instance it's the only site I've encountered where people often post extremely funny comments despite the serious topics. The moderation system there encourages that sort of behaviour.