Upgrading your existing Drupal 7 sites is strongly recommended, especially if you encountered problems with Drupal 7.20. There are no new features in this release.
Drupal 7.21 is a bug fix only release. The full list of changes between the 7.20 and 7.21 releases can be found by reading the 7.21 release notes. A complete list of all bug fixes in the stable 7.x branch can be found in the git commit log.
See the 7.21 release notes for details on important changes in this release.