What does this mean? Well…good question. For you, the end-user, not much, though as MySQL is supposed to be a bit faster, posting comments might be a bit more responsive than it was before. For me as the author/administrator, it should mean a bit more speed when creating/editing entries, and a lot more options as far as what I can do on a design/implementation level.
At least, that’s the theory. If nothing else, according to one of the moderators of the MovableType Support Forums, “If you have the choice, pick mysql. :) A little faster, more stable, easy to browse the database….” Good enough for me!