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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.
Users can now search for apps in Launchpad. There is a small search bar at the top of the window, allowing you to search for your apps in real-time.