A lot of the quality degradation stems from the mistaken idea that everyone has something of value to contribute. They don't. Discussion often involves a handful of quality posts, and then a bunch of "I agree"'s clogging it up. Posts that don't add to the discussion should be moderated, or, at the very least, targeted for downvotes.

Of course, it'd be tough to gain traction with users who believe their karma score matters. Better to hide it from them completely and indicate scores in another way.