A good deal of OS X is open source, but that's beside the point. I don't really care that XNU, etc, are open source, because I don't want to develop an OS -- I want to develop for that OS. If you're so bothered about everything being open, then I suppose you are using OpenBSD, not Linux or anything else, because it doesn't contain binary blobs? Seriously, I don't understand why you're so bothered about things being open that you'd never change anyway.

On the other hand, Cocoa is a better API than anything available for Linux or Windows, and that does matter to me. So do the consistent and logical keyboard shortcuts across all apps, the high level of UI design absent on almost every other OS, how all my hardware just works without having to edit configuration files, and loads of other detail that Apple pays attention to. There's a lot more to good design than being grandma-friendly, and not many people except Apple get it right.