It's a mess because there are several ways to do the same things, and no two things are done in the same way. It feels like any time I want to do something new in OS X I need to learn a new system of some kind. That is bad design. It should just be obvious.
If "obviousness" is the criteria for a well-designed OS, I'm not sure we've seen a (successful) well-designed OS in the history of computing. OS X certainly doesn't deserve to be called out. :)
Actually, I think it does. OS X is a horrendous frankenstein of several different systems with different design philosophies. It is really, truly awful. (Although it works - I'm using an OS X machine to type this now.)
One of the great things about Plan 9 is that everything implements the same interface. If you can interact with one thing, you can interact with everything. Using new parts of the system becomes obvious because you already know the interface.
A concrete example of why this works: In Plan 9, process information is available via (guess what) the file system, so the Plan 9 debugger just reads the state of running processes from those files. Because file systems are automatically exported over the network (also part of the file system), you can debug a running process on another machine without the debugger knowing anything about the network. Nobody had to implement network debugging - it just worked right away because of good design.
You know you have a good design when these kinds of complex behaviors just "fall out" without any additional work.
