On Slashdot, the moderation system keeps good comments at the top and bad comments hidden. This is why the quality seems so good: one only has to read the top of the comment page to get really good discussion. So, regardless of how many trolls there are, they remain out of view. This was Slashdot's greatest innovation, IMO.
Reddit deals with the same issues: plenty of smart users, so they need a good ranking to keep the good comments at the top. Reddit used to use Slashdot's approach to ranking, but the inherent moderation system is different so it didn't work. The average comment in an active story on Reddit can get dozens of mods VS less than 1 on Slashdot. Reddit's problem was Slashdot's system heavily biased in favor of comments with a lot of moderation (upvotes minus downvotes is scaled higher). Typically, the first few non-troll comments were fixed at the top. On Slashdot this isn't a problem because mod points are rare so people use them with more care; also, the maximum score is capped at 5.
Reddit recently started using a more statistically sound approach which rewards high upvote:downvote ratios, and the comment quality has drastically improved. It saved the site, IMO. Slashdot is still known for having better quality comments than Reddit, and I commend them for it.
See http://blog.reddit.com/2009/10/reddits-new-comment-sorting-system.html for more information on reddit's new system.