There's no money in quality comments. Reddit is a vast money-pit for Conde Nast. Kevin Rose might have got paid, but Digg is going down the chute. HN serves as a recruiting ground for YC and I believe that Slashdot is treated as a loss-leader to support other Geeknet properties.
Personally, I think we need to take a lesson from the granddaddy of them all, WELL. Mandate real names and charge admission and you'll get quality interaction. Contrary to what internet culture might have us believe, exclusivity and accountability are generally good things.