Allows fancy PDO database queries (foreach through a result-set, select columns of result-sets, etc)
Connect to multiple databases simultaneously
Database replication
Type-agnostic prepared statements, so we don't need to bother with %s or %d
Proper support for NULL values
Adds support for transactions, while allowing them to not die on non-transactional databases
Support for "INSERT ... ON DUPLICATE KEY UPDATE ..." on MySQL or whatever the database-specific equivalent on other databases
Support for multi-insert statements, on all databases, using the appropriate database-specific mechanism
"INSERT DELAYED" support on databases that support it or some equivalent, which should help performance on logging and similar "don't need it back immediately" queries
Fully-structured INSERT, UPDATE, and DELETE queries with a simple fluent API (chainable, like jQuery) syntax
A Fluent API query builder for SELECT statements that weighs in at only a few hundred lines (not counting comments)