In addition to the above mentioned changes:
- hook_user() was changed to allow multiple pages of user profile information. The new syntax of the hook is given in the API reference. Pay particular attention to the "categories", "form", and "view" operations.
- When processing a form submission, you should use drupal_goto() to redirect to the result if the submission was accepted. This prevents a double post when people refresh their browser right after submitting. Messages set with drupal_set_message() will be saved across the redirect. If a submission was rejected, you should not use drupal_goto(), but simply print out the form along with error messages.