Yep, from what I've read - but I'm no expert - launchd sounds pretty good.
The core of the OS, darwin, is also, open sourced - there's even an x86 version :wink: - you can read about it and download it on the Apple Developer's Site
With regard to the structure of the file system, it's completely different in OS X to the majority of other Unix based systems. Not only that but is uses the NeXT based package system for most of the Mac OS X applications. For example, you'll find the Mail program acutally runs from Mail.app, which is acutally a special kind of directory that contains all the config and code to run the application.
Having said all that, user directories don't normally sit in the root filesystem. They should be in the user's home directory, which should be somewhere like /Users/<username> but I can't quite remember!
For a start, the Mac doesn't use X11 as it's windowing system, it uses Quartz and Aqua which are the proprietry components, or at least some of them, that make Mac OS X closed source. X11 is available for Mac OS X though, it's part of the develper package. If you install the developer tools from the OS X cds, it should be there.
Phew, far too late to go on about this any more without firing up my mac. Hope it's of some interest.