I think a lot of the growth problems might have to do with the devaluation of karma. The solution to this might actually raise the quality of comments and front page stories.
When I posted a comment here a year and a half ago, I felt like each vote was a real, actual smart person praising or agreeing with what I had said. Each point had a lot of meaning, and back then 5 points was considered a pretty good score. Now each point means much less for some reason. I'm not sure why. Maybe it's because I don't trust each individual person as much. For example, as I write this, the top comment is about someone using a script to submit this article. It's two sentences, and it has received eleven votes. To me, each of those individual votes mean less than they used to.
The net effect of one vote, for the whole culture of hacker news, has gone down significantly. This reduces the total perception of worth for each user, and, more importantly, for each voting action. Back in the beginning, I felt like I had some amount of power, in that if I voted something up it would literally rise in the list. In fact, I felt like I had some sort of responsibility. Now that active, obvious change in story rank rarely happens, so I feel like my vote is somewhat worthless.
I tend to mostly agree that the quality level of submissions and comments has stayed the same during the growth of this site, but the devaluation of karma has led to other cultural changes. There's something comforting about sharing ideas within a small group of people because you feel like you have real influence, and I think I'm starting to lose that feeling here. There are so many new faces that my perception of quality has gone down, even if the actual net level of quality for each item has stayed the same. Seeing many posts with 100+ karma on the front page recently has kind of solidified that feeling for me. Who are these hundreds of people?