This is belated...anyway, I'm taking a day off work to get ready for our vacation this weekend, and I remembered (after writing some other blog posts) that I meant to chime in on a Planet Debian discussion about closed vs not-closed blogs.
My reason for closing comments was quite simple: blogs are spam-magnets. While I was a student and/or unemployed, I didn't mind spending a few minutes a day purging spam messages from my blog. But when I started working full-time, it became a real chore, especially since the amount of spam I got increased with the number of posts on the blog.
One thing that would change my point of view here would be the option to automatically close comments on posts that are more than (say) 60 days old, or perhaps that haven't got a (non-deleted) post newer than 60 days. This would keep the spam problem down, since my biggest problem was ancient blog posts that no-one had commented on (at least for months or years), but that suddenly got picked up by spammers. This might be available if I ran my own blog, but I don't have the time at the moment to administrate a home server, so I'm stuck using someone else's software.