So, someone had a great idea to go back through the Developer Zone, which has become a bit disorganized of late, and put together an article with links and summaries for all the Dev Zone articles that discussed MySQL 5.1 features. I have done so, and also linked in many community member's blog articles and tutorials to give everyone a good dose of the features that make MySQL 5.1 worth investigating.
The main features that MySQL 5.1 brings to the table are table logging, events, partitioning and a new form of replication called row-based replication. However, there are many smaller features such as XML functionality and essential stability and bug fixes that were mentioned by the community that are very interesting.
The article continues at http://dev.mysql.com/tech-resources/articles/mysql-5.1-recap.html