Nilay Patel also has a good overview of Mountain Lion for The Verge, and a includes a good video overview as well.

Regarding Gatekeeper, and what the default is for what apps you can install:

Mountain Lion will ship out of the box restricted to running only signed apps and apps from the Mac App Store. (You can still run any app you want by right-clicking on an unsigned app or simply changing the global setting to allow apps from anywhere. You can also lock things down even tighter and only allow App Store apps to run.

The Verge’s Mountain Lion Preview