The Registration screen/process has a few kinks:
After entering username and passwords, and scrolling a mile to the Accept button, and then submitting, THEN the system tells you that the username you chose has already been used -- AND THEN it brings you back to the Register screen -- WITH ALL FIELDS EMPTY!
The system should check the username IMMEDIATELY, and not let you go further. If it's already been taken, you should be told, and immediately put you back to choose a new name.
Same for captcha errors.
Think about what happens if the user selects 3 or 4 names in a row that have already been taken - you expect they'll have a good experience having to re-enter everything 3 or 4 times?
Do the verifications immediately, not after they've entered all the fields. Don't make the user go through re-entering everything each time.