There are many “under the hood” changes, but the most obvious changes are adding Automator support, which means Mac OS X 10.5 “Leopard” is required.
Additionally, the options for what to do when an application is being used have been simplified. It turns out it was never useful to automatically unmute sound when the DVD Player is in use.
As always, updating is very strongly encouraged during the beta period. I try hard to make the latest build work, but I do not support old beta versions.