In two words: online operations. In a paragraph: Forget partitioning, row-based replication and events. The big reasons most people are going to salivate over 5.1, and probably start plans to upgrade now, are the online operations:
- online ALTER TABLE for column rename, column default value change, and adding values to the end of an ENUM/SET
- Online, table-based logging. No more need to restart your server to enable or change the general or slow query logs. You can have the standard file-based output or choose a table format…which you can query.