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An overview of which features were added in MySQL 5.1 can be found here: Section 1.4.1, “What Is New in MySQL 5.1”.
For a full list of changes, please refer to the changelog sections for each individual 5.1 release.
For discussion of upgrade issues that you many encounter for upgrades to MySQL 5.1 from MySQL 5.0, see Section 18.104.22.168, “Upgrading from MySQL 5.0 to 5.1”.
For changes relating to MySQL Cluster NDB 6.x, see Section 17.7, “Changes in MySQL Cluster NDB 6.X and 7.X”.