Sure enough, there was a post in the forums from WordPress staff member “ajmattic” (now probably hiding somewhere in a witness protection program):
The reported behavior of email notification for comments is not a bug. It is a new default setting when commenting.
This feature was a recent change to the way comments work on all WordPress.com blogs.
Once you click in the Leave a Reply comment box, you will see the “Notify me of follow-up comments via email” check box appear. Simply un-check this box before you submit your comment and you will not receive email notifications.
If you ever forget to un-check the box you can always unsubscribe here:
Actually, ajmattic’s comment was already there when I posted my comment on the thread; I just didn’t see it. So yes, our worst fears are realized. All that garbage I wrote about yesterday is exactly what WordPress intended.
People on the forum thread have indicated that the majority of their commenters do not request email notification about new comments, and personally, it’s something I very rarely do. Therefore I’ve changed my settings so that my readers will no longer have an option to follow comments via email. This means you won’t be getting a lot of unwanted notifications from this blog. I apologize if this inconveniences any of you who like the notifications, and if several of you mention below that you’d like me to restore the option, I will.
To disable the option on your blog, so as not to spam your readers, go to Dashboard > Settings > Discussion and on the Discussion Settings page, scroll down to “Follow Comments” and uncheck (or leave unchecked) the box saying “Show a ‘follow comments’‘ option in the comment form.”