What's different from the current OS apart from the rounded corners for the context-menu?
Complete configurability without the need for any 3rd party patches.
Basically, they're including Birdie, VisualPrefs and Magic Menu into the OS while adding further configurability (have a closer look at the screenshots and you'll see that you can even select background patterns for scroll gagdets).
The GUI will be completely
skinnable, but that funtionality may not be ready for OS4.0, it will probably be implemented in a BoingBag or OS4.1.