Mach and IOKit and and a gazillion other things are hardly "the same old Unix stuff", if you're talking about the kernel. If you're talking about everything above that level, read Amit Singh's book if you really think there are just splashes of superficial changes at and above the POSIX layer. If you have and you still feel this way, I don't know what to tell you. I'm not saying it's all great, but there's a LOT there.
OS X is BSD with a fresh coat of paint. It really depends if you want to consider the whole experience (which I consider less stellar than most people would care to admit) or the internals (which are the same old unix stuff).