When you download Minecraft on a default-configured Mac (Gatekeeper on) and try to open it, the system will report that the application is "damaged" and cannot be opened. This cannot be overridden by using Finder's Open contextual menu item, as is possible with unsigned applications. This is because the launcher is signed, but the code signature is incorrect:

$ codesign -vv Minecraft.app
Minecraft.app: invalid Info.plist (plist or signature have been modified)
In architecture: i386

The solution to this issue is for Mojang to re-sign Minecraft.app after making any changes in the .app bundle. Disabling Gatekeeper (as the official support article recommends: http://help.mojang.com/customer/portal/articles/901687-error-minecraft-is-damaged) is only a workaround and leaves systems significantly more vulnerable to malware.

Relevant docs:

• https://developer.apple.com/library/mac/#documentation/ToolsLanguages/Conceptual/OSXWorkflowGuide/DistributingApplicationsOutside/DistributingApplicationsOutside.html
• https://developer.apple.com/library/mac/#documentation/Security/Conceptual/CodeSigningGuide/Introduction/Introduction.html