Key features in MySQL 4.1

MySQL 4.1 introduces many long-awaited features, such as subqueries, Unicode (UTF-8) support, support for character sets and collations, as well as GIS data types for manipulation of geometrical data.

MySQL 4.1 features one of the probably longest-awaited features in MySQL besides the transactions - subselects. Implemented are value-returning (single and row value) and boolean (EXISTS, IN, ALL/ANY/SOME) subselects as well as derived tables.

Version 4.1 also has greatly improved support for character sets. Introduced is Unicode (UTF-8) support, but now it's also possible to specify character sets per column, table and database. This should make MySQL much easier to be deployed as a backend for multilingual websites and applications.

GIS support allows MySQL to be used to store geometrical data.

The new SHOW WARNINGS statement shows the warnings generated by the last SQL statement executed. Among other things this makes tracking down errors in large bulk inserts much easier.

These are by far not the only enhancements and improvements in 4.1. For a detailed list, see the ChangeLog entries in the MySQL manual:
http://www.mysql.com/doc/en/News-4.1.x.html