[Edit: Issues, Bugs, and Requested Features should be tracked at Google Code, not here -- matt, 2010-04-23]
Less Wrong is still under construction. Please post any bugs or issues with Less Wrong to this thread. Try to keep each comment thread a clean discussion of each bug or issue.
Requested features... sure, go ahead, but bear in mind we may not be able to implement for a while.
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 :).
There's plenty to improve about the registration page. I've logged an issue for this particular suggestion.