Gatekeeper is a Good Thing. It has a reputation for being about restricting apps you can run on your Mac, which is unfair. I leave Gatekeeper enabled: if I try and run an app that isn't code-signed, all I need to do is right click and click "open" and accept the warning.

As others have said, ML is solid. All consumer features can be safely ignored: don't like iCloud documents? Turn it off and use your Mac as you always have. Under the hood, ML is as good as the Mac has ever been.