Why don't I allow comments on most of my posts? Part of the reason is the 'high level' of discussion that tends to occur in threads attached to posts that address 'hot button' issues. A good example is the 'commentary' elicited by Why Sexism is Obsolete over at Victor Reppert's place. Not an edifying spectacle.
Curiously, the lack of comments does not seem adversely to affect my traffic. In fact, I have e-mails from people who positively like the paucity of comments.