Quote from Monty's blog:
The main reason for leaving was that I am not satisfied with the way the MySQL server has been developed, as can be seen on my previous blog post. In particular I would have like to see the server development to be moved to a true open development environment that would encourage outside participation and without any need of differentiation on the source code. Sun has been considering opening up the server development, but the pace has been too slow.
I would hate to see MySQL turned into ashes.