I believe the decrease of 'quality' is due to the failure of HN to create a society of like-minded people. This failure is on two levels. Firstly, the open voting system and comments drive has a tendency to revert to the mean. Secondly, HN needs to create hackers the way a school creates students. I realize there is an anti authoritarian streak among hackers but a geek club is pretty exclusive in its taste. New members have to be "schooled" into the ways of a hacker. In real life, it is impossible to have a town hall meeting where everybody talks at once, but HN is already bigger than a town hall.
I believe Quora does rather well in this respect because it encourages longer, considered posts. The (fast) rate of decay on the front page partially contributes to the problem because it models a news site, rather than a technical discussion site, where most techniques and approaches remain timeless.
Here are some possible approaches:
1. Encourage longer answers and comments at the top level. This can be either implemented as a simple word limit, or automatically placing longer comments at the top of the comments list.
2. Recycle old posts which have good comments. This should fix the disincentive for people to provide long-lived answers.
3. Make HN a "not" news site. This means that the incubation period is longer before posts make it to the front page. Unless something has a long term value, it will less likely be voted up because the reader would have already seen and discussed it on TC, Reddit, Digg etc..
4. (option to #3). Have posters classify whether the post is a news or a technical discussion one. News links will have a different rate of decay, and will occupy limited number of spots on the front page. Furthermore, these posts will not be recycled.
5. Require a link to be submitted with some comments. This is to encourage submitters reason like hackers do. Provide some guidance - e.g. does this news contain some data? What are the insights/inferences one might draw from this? Does this article discuss a problem domain? Does the post illustrate an assumption that is subject to hacking? What is your personal take on this? It also acts as a disincentive for people to submit links without giving the topic due consideration. I recall that eHarmony was very succesful in its early days because internet dating sites usually have more men than women. By subject the men to a barrage of interview questions, eHarmony was able to maintain a balance between the male and female participants. I thought this was a great hack.
6. Implement some sort of disincentive for upvoting of inane comments. For an example, do an automatic Quora-style follow, where you will start to see this person's comments at the top of the comments page. Make it difficult to "unfollow" (say three clicks). It will encourage people to be more careful about polluting their personalizations.