Marco Arment, Tumblr’s lead developer, on comments:
I also disagree with the widespread notion that comments are “discussion”, or that they form a “community”. Discussion and communities require mechanics such as listening and following up that are rarely present in comments.
A blog post is a one-to-many broadcast. Comments are the opposite: many-to-one feedback. A true discussion medium would encourage more communication between the commenters, forming larger quantities of many-to-many interactions and de-emphasizing the role of the blog post’s author. In practice, that rarely happens.
If comments are behaving as many-to-one feedback, there’s minimal value to showing them to the world, because the world largely doesn’t read them. But the act of showing them to the world — your world, not the commenters’ — creates a setting in which commenters are encouraged to behave negatively.
I emphasized the first paragraph because for me that’s what really hits the nail on the head.