Drupal 7 introduced a better and more secure way of hashing passwords. This method has been backported to Drupal 6 in various modules, including the Secure Password Hashes and Password modules. When upgrading from Drupal 6 to Drupal 7, core assumes that passwords are always stored in the old format, and people are unable to login anymore.
Core should just skip passwords that are not stored in the expected format: a hexadecimal md5 hash.
None. Note that this patch applies only to Drupal 7, not Drupal 8.
User interface changes