This release contains many improvements, new features, bug fixes and optimizations. It has not undergone testing at scale and should not be considered stable or suitable for production. This release is being classified as a minor release, which means that it should be API compatible with 0.20.2.
See the Hadoop 0.21.0 Release Notes for details. Alternatively, you can look at the complete change log for this release or the Jira issue log for all releases.