I realize that doesn't directly relate to comments, but I think some of the declining quality of conversation owes to the fact that it's getting a bit stale. How many blog articles about productivity can we discuss without some decline?
I don't think we should ban political articles at all. In fact, I think less blog posts about "are we in a bubble?" and more articles on economics, science, philosophy, etc would make HN much more interesting. The median comment here is still of much higher quality than at sites like reddit. And although certain subjects can be sensitive, I doubt that banning these topics will actually reduce meanness, it will just make the change in decorum harder to notice.
Finally, a more extreme idea: why not add a second kind of vote? Perhaps we could vote comments agree/disagree in addition to up or down. These could be right and left arrows, to drive home the point that disagreeing with something ought to be orthogonal to whether it adds to the conversation. We could weigh these votes less, so that rankings more reflect how insightful we think something is, instead of how popular.
To use a person anecdote, when I come to HN I immediately look to the top. I have come to expect good stories up on top, and I'm sure I'm not alone in that sentiment.
This being a likely user behavior, you could game that behavior by putting 5 or so of the newest stories in a thinly bordered box, above the normal top stories. That way when people go looking for the top stories, they see the new stories, and are more likely to read and comment on the interesting ones.
This will preserve the normal UI of the site, with minor UX changes, while giving exposure to each story as it comes in. If I were to guess, most of the good stories that come in and languish do so because of a lack of exposure. I'm willing to bet that most people don't look at the newest page very often, expecting others to upvote the good stories onto the frontpage for them.