If every process under Mac OS X, Linux, Solaris, HP-UX, etc. ran with uid=0 and every directory and file under /etc, /usr and /var were given 4777 permissions and almost all applications could execute arbitrary /bin/sh, perl and Python scripts silently behind the user's back, you'd have the same security mess that Microsoft faces.
Uhhhhhhh.... yeah, what he said.
EDIT: I'm not suggesting it would be more secure, just that Windows has the advantage of user familiarity and Unix systems enjoy a vague public perception that they are somehow better/more secure than Windows, although more or less unattainable for, say, My Mom. The unholy alliance of the Windows brand with a Unix Kernel will either negate the existence of the universe, or it will create an environment where people ("My Mom" people, not "us" people) are excited to buy Windows because they think they are getting a super-awesome 21st century OS that is comparable to OSX, rather than how they seem to feel about Windows "Lipstick-on-a-pig" Vista.
It's all about marketing and perspective, your /sqrl and /whatsit are powerless next to My Mom's perception of her OS choices.