What common protocol? Mac OS X doesn't have dbus. It recently acquired XPC, but XPC is NOT a system bus -- it's simply an IPC library. It's neither required, nor universally used, nor is it terminally glued to the remainder of the OS.
> You don't, for example, just replace launchd with some other init system.
You don't? Says who? All launchd does is serve as an inetd/cron-esque daemon. The system daemons themselves continue to vend standard sockets and mach ports, and launchd itself does not pervade their externally vended interface.
> You don't just say "hey, screw Aqua I want to use something else."
Actually, you can. The display stack is driven by IOKit; XDarwin can run directly atop said stack.
Likewise, none of the "Mac-like" features that you want -- for example, automatic network configuration -- rely on any sort of centralized entity that pervades all daemons. There's a seperate system -- SystemConfiguration -- that provides that functionality, abstracted through a set of well-defined APIs and modular and distinct from the remainder of the system.
> * It's a collection of daemons running that communicate over a common protocol.*
Hence it's really rather monolithic, because to play in the systemd universe, everything has to conform to systemd's tightly coupled design.
