From my limited knowledge: The Unix OS coder assumes the user knows what they're doing, etc. versus the Apple's you can only do it the way you are allowed to do it, etc.
Compared to other variations of Unix, what are Mac OS X's limitations, but also does it improve the user experience?
Edit: To try and clarify the question, it's more the user experience I'd like to read find out about. Although it's of interest that it's certified, that's not so relevant - Linux is not certified, but it's the closest I've come to using Unix up till now.