When commenters use their email address to comment, it says that the email is associated with a WordPress.com account, and that they should log in to use the email.
This problem has directly arisen as a result of your change implemented as per 3/15 here: http://en.forums.wordpress.com/topic/recent-update-to-commenting?replies=1
The problem is that people shouldn't have to log in to comment -- especially if a blog author does NOT have the "Users must be registered and logged in to comment" setting turned on.
For example, this change is a great slap in the face to Gravatar users, who are explicitly used to having their gravatar follow them around on sites without having to sign in, simply by virtue of the use of their email address.
Please see also: http://en.forums.wordpress.com/topic/you-must-be-logged-in-to-comment?replies=87 for others who are having the same problem.
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