One way to promote quality is to crack down on low-quality content (which is something that HN does and you've done a good job at doing. :P).

For example, many popular subreddits support the making of image macros/memes. Many of the serious subreddits have a) banned the use of image macros/memes and/or b) forced the sub to use text posts only, which forces the submitter to add discussion, and also reduces the incentive for karma-gaining since text posts generate no karma.

Another approach is to promote original content [OC], which gives an incentive to submitters to submit unique content instead of just being the first to post a submission to a new article on TechCrunch for internet points. The subreddit /r/dataisbeautiful has done this very well. (I really wish Reddit would remove its 10:1 rule, which was made to punish self-promoters and is a different issue entirely)

And of course, the standard machine learning techniques can be used to predict the probability of a post being good given, for example, a) keywords in title b) quality of domain's previous submissions c) quality of user's previous submissions, etc.