Recent enhancements to MySQL have improved its performance and usability considerably. When it comes to distributed usage, though, MySQL has still a few bottlenecks and constraints that make the DBAs and developers lifeharder. For example, while MySQL itself is multi tasking, the replication apply, which is core to a large chunk of the web economy, is single thread.
Another example is the singular combination of master and slave. Each slave can only have one master, making the developers quite unhappy, as they have to stretch their imagination to get things done. Both the above constraints, and a few more, can be removed easily with Tungsten replicator, an open source replacement of MySQL native replication, that takes your development and administrative experience to a whole new level.