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Lost mode also works in "Mountain Lion", just as it works in iOS 6. If you lose your Mac, you can send it a message. You can also remotely lock your Mac in true iOS-style, and even remotely wipe the device from iCloud.
Gatekeeper is a new feature to "Mountain Lion" allowing only Mac App Store and/or identified developers' access to your Mac. If the developer is not registered with Apple, or the application was not downloaded from the Mac App Store, it won't run unless you change the settings.
You can see here, an app called "Cocktail" was not downloaded from the Mac App Store, meaning it simply won't run. You can always change this setting, but be careful.