As I read from Brian Aker, the beta freeze for MySQL 5.1 is right ahead. Time to reflect on the features that I know they will be in 5.1, features that I don't know if they're in 5.1, features that I wished they were in 5.1 and features that will very likely not be in MySQL 5.1. There are still some questions open for me - maybe somebody who knows more can answer some of them ;-).
New features that I know for sure are Partitioning, which allows to spread the logical representation of a table on multiple physical storage locations, based on conditions. Row based Replication means that data can be transfered from the master to the slaves row by row rather than by SQL statements (it will be adjustable, which method to choose). The Task Scheduler will allow (as the name says) to schedule tasks (one-time or recurring) to execute Stored Procedures. This will for example make it possible to plan clean up tasks (like removing invalid data). This feature might be extended to also allow setting automatical alerts on specific events. A big new feature will be Plugable Storage Engines. Developers will be able to write their own Storage Engine and plug it into MySQL - it could become very exciting which new Storage Engines will come up and provide great new possibilities. New features in MySQL Cluster will be Disk Based Storage, Global Replication and Variable Sized Attributes.
Another very interesting feature that I wrote a feature request for is logging into database tables. Currently, log files are written to the file system and I thought it would be nice if they could directly be written to MySQL tables, because this would make it much easier to analyze the log data.
There are features that I expected to be in MySQL 5.1, but I haven't heard about them for quite a while, so I hope, they haven't been postponed and there's still a chance to see them being implemented in 5.1. This begins with Check Constraints, a feature which I think is long overdue. Another very important feature would be privileges for trigger creation and deletion. At the moment, these actions require the SUPER privilege which should only be granted to database administrators, so there's no easy way to let non-privileged users create and delete their own triggers. Also Exception Handling in Stored Procedures is a feature that quite many people still miss.
Foreign Key Constraints in MyISAM tables seem to have been postponed for MySQL 5.2, as the MySQL manual says. FK constraints in MyISAM tables would be especially useful because it's not possible to enforce referential integrity and use fulltext indexes in the same table. And as I recently wrote, it would be useful to have a command that checks referential integrity, after foreign key checks have been set to off.
Another feature that's planned as far as I know, but probably not very soon, is to provide other languages for Stored Procedures, like C, Java, Perl, PHP, etc.
I would be greatful about any piece of more detailed information as I could provide here, so if you know more, please leave a comment or mail me at email@example.com.