We actually did make our "own" OS in java when I was taking CS in java. It was a unic CLI clone with EXTREMELLY limited functions ( cat, ls, pwd, edit (make a text file), mkdir and cd being a few of them) and it was a lot of code. I cant imagine someone actually making a GUI based OS by themselves, its quite an accomplishment ;-). I have been lurking around a site called freeos.com and there is one x86 OS called AtheoOS. Even though there is a development team on it at the current phase of development it started out as a one guy project! Just leaves me in aw.
A friend of mine and I wrote a graphical shell for launching programs as part of a programming contest. To date the contest we were coding in BASIC *shudders*. Probably not what you wound call an OS all it did was find all the apps and present them in columns that you could key through and press return to launch. It ran on an Apple IIgs.
I've used OS X for 2 weeks, but it's too quirky for basic daily use. I need to reboot to run my homebanking software (which runs inside Virtual PC), Napster is very unstable under OS X and the most important: Steinberg's Cubase VST won't run. And the Finder has lots of inconsequencies in it.
And then, when I rebooted in OS 9, I was very impressed by the responsiveness of that good old system I had started to take for granted.
And oh, the Dock takes unnecessary screen space. Under OS 9, I'm running an extension called Taskmenubar. It does exactly what the dock does, but it's always visible without taking screen space. It runs inside the menubar.