The register page should explain what is a valid username.

If I enter an invalid username, it should tell me what is invalid about it (instead of only displaying "Invalid user name", making me guess).

For a checklist for usability issues, I recommend a book: Defensive Design for the Web: How to Improve Error Messages, Help, Forms, and Other Crisis Points by 37Signals. (Each recommendation made in the book is pretty obvious; the purpose of having a checklist is to remember to do all of them :).