[spicytunacone]

Another failing of the UI/UX on some social news sites is not being able to CC other posts. If there are notifications, alerting only the parent of new activity only does so much.

While sdrothrock is correct that these sites don't have much incentive to produce quality discussion, I do agree with you that there's value in seeing what gets votes as opposed to what gathers replies (not that they are necessarily mutually exclusive and not each plagued with their own problems). I wonder if it's worth trying a system that marries the two elements. Take a conventional BBS, for example. They often have a reputation system that in the best case encourage quality contributions and avoid posts only consisting of "good post, I agree." However, they can't be used to indicate from the index what discussions are worth treading. You can try highlighting the top voted comments[1] beneath or side-by-side their respective links to the OP. Then you can infer what kind of discussion is taking place. Is it a bunch of metajokes or did someone just unload a lot of expert knowledge?

For news aggregators to facilitate enduring discussion is trickier as the nature of news is to remain current. Maybe if a link would otherwise decay but discussion is ongoing, the news entry on the index could take a backseat to its most 'active' threads. Indented new lines underneath the post that link to those threads, "The conversation is still going. Click to expand" or something like that.

[1] Perhaps highlighting only a number of posts proportional to the total number of comments or only the posts above a minimum score that is proportional to the total number of comments (just so you don't get highlighted for being one of the first responders). It's also probably important to not be able to vote on the post from a preview, so people have to click into the thread and hopefully read it and possibly contribute.