It turns out it is possible to configure Wordpress to make leaving comments almost impossible. Without realizing it, I had done just that here at zanshin.net. The configuration has been updated, so any one should now be able to leave a comment or three, if they so choose.
General Options: Membership
On the Options page, for the Membership setting, I had “Users must be registered and logged in to comment” checked. It turns out that this required anyone desiring to leave a comment to register themselves with my site. Not a bad idea, in and of itself, but one that created a roadblock. Prospective commenters were presented with a log in page, no instructions, and a tiny “Register” link to follow. By turning this option off, visitors now only have to provide a name and email address in order to comment. And I still, through the Discussion Options, get to moderate comments before they appear on my site.
The lesson reinforced here is to test everything and assume nothing. Had I not asked Sibylle to leave a comment (to test a feature of my site’s statistical package), I never would have known that my combination of configuration choices had made commenting next to impossible.