Jeremy Heigh makes an interesting observation about a recent thread on kottke.org, which I think applies broadly across the blogosphere:
...we were trying to understand how to better leverage all the great, individual thinking being done on blogs because what Kottke hosted wasn't a conversation at all. It was nearly 80 people carrying on their own conversations with themselves while others watched. That's not a conversation -- that's philosophical voyeurism spiced with a hint of insanity.
I think choice of topic, the way in which the question is posed, and the pace of the commenting[1] has a lot to do with it. Despite the large number of comments, there are some threads on kottke.org which have been more conversational in nature and from what I hear, there's still the occasional MetaFilter thread that's worth reading for the conversation. But as Jeremy notes, there's still a ton of chaff out there obscuring the wheat.
[1] Sometimes I'll get 30 responses to a post in the first hour...that's one every two minutes. That can be a velocity and volume not conducive to conversation.