I’ve received a complaint about difficulty commenting here, and from looking around, other WordPress.com accounts are having the same issue. What happened is that WordPress changed the commenting system somewhat, so that anyone who has been using Gravatar (which is a part of WordPress) has to “log in” to comment. There was no real announcement of this, it’s buried down in the “support” threads:
We’ve recently updated our commenting system.
Now if someone tries to comment with an email address attached to a WordPress.com account (or Gravatar account, they’re the same), they’ll need to sign into WordPress.com before they can comment.
If you’d like specific help, please check out Diary of Daedalus who has a nice (with pictures) step-by-step walk-through. Sorry for any inconvenience, apparently this was something WordPress thought up without asking any of us. 🙄