what UI changes are still needed before public release?
Definitely the Icons.
The idea with the 'flat look' isn't bad, but the Icons should be more elegant, filigree - currently it looks a little bit like painted by a child. Compare e.g. the analogue clock in the System Preferences or the Dashboard Icon between 10.6 Snow Leopard and 10.10 Yosemite PB3 ....