On the Mac, you see it all the time. Because it is actually designed so that it can be seen. The only reason you want to hide the directory structure as a low-level OS implementation detail is because you can't do anything else with it! It's so hopelessly convoluted that it becomes a low-level implementation detail, instead of a way for the user to interact with his computer.
To be exact, though, OS X lives a bit of a double life - it contains both the legacy Unix structure, which is hidden, and the new Mac OS structure, which is shown.
You misunderstand. OS X does not rely much on the legacy structure. You could probably delete it entirely and the OS would still run just fine (although likely not boot). It really is legacy, in that most Mac OS apps do not use it. They use an entirely different hierarchy, which is very much visible. Applications, libraries, OS components, they all live in the new one.
The same is not true of any other Unix system I know.