Nelson Minar has an interesting post on username restrictions in web applications (via rc3.org). I agree that email addresses are better than usernames, as they can be remembered easily, rarely change and are usually unique to an individual.
The only thing Nelson didn’t touch upon which I feel is important is the ability to use OpenID as well as a standard username/password combination. A major irritation is having to go through the whole sign up process for a site just to add a comment or report a simple documentation bug.
Incidentally, Pub Gateway allows both OpenID and email address logins, with a display name of your choice — though the display name has to be unique to prevent confusion. I must admit that I haven’t tested Unicode support, though it’s on the list of things to check on the site.