I just created a new account on my blog. Security is a concern, so I wanted a username that would be hard to guess. It took me over 15 tries before I got a name WordPress would accept.
To make matters, worse, WordPress's administrator interface for entering new users blanks the password field each time it reloads the page with the error message. That requires re-entering the password, twice, every time there is a username error.
WordPress's unhelpful error message:
ERROR: This username is invalid. Please enter a valid username.
How can a user or administrator know what the problem is? We are supposed to guess?
The error message needs to provide enough information that the user can figure out what the problem is. In this case, it needs to list what characters are valid, and any other required parameters such as minimum and maximum length.
Better yet, provide this information on all registration screens. That includes both the user registration window, and the the Administrator panel for adding new users. That will help users and administrators pick valid usernames and avoid these problems.
I'm surprised these problems remain in an otherwise excellent interface. Especially since they affect -- and discourage -- NEW WordPress users. Please fix!