While I almost flipped off my chair when I read the post, I realised that the very same question was in my mind when I started off on my Collie 1~2 yrs back and I'm sure is in many newbie's mind. So its a good question to be answered.
As mentioned Meanie, OZ is the kernel OS while OPIE and GPE are the GUI shell for OZ. So ...
*NIX
----------------------------------
| Apps |
----------------------------------
| OPIE | GPE |
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| OZ |
| (include drivers) |
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| Hardware |
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Windows world
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| Apps |
----------------------------------
| Windows OS |
| GUI / System libraries |
| (include drivers) |
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| Hardware |
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Granted, Windows XP itself also have its own separation of system (MSVCRT) & gui (GDI) layer and drivers (WDM) layer, including DirectX (HAL/HEL layers) which provide a slightly more direct access to the hardware (hence from what I know, the "DirectXcess" name) ... the main difference is that the Windows shell (GDI) is the *only* shell available for each version of Windows. Users cannot choose to load other shells from 3rd parties and Microsoft itself do not offer other shells for GUI.
The downside is that it is deemed as a closed and monolithic approach to OS architecture whereas Linux is deemed more modular and open. I do however remember that in the early Win3.1 and Win95 days, Microsoft was often critized for having just slapped a GUI shell onto MS-DOS and try to sell that as a GUI OS, and yet when Win98 and subsequent versions integrate the two closer, MS is faulted for being monolithic.
Of course MS is also not an Angel and not entirely faultless. It could have made it available for 3rd parties to write complete GUI shells with its own messaging system and libraries akin linux with its different shells but still have a single X11-like GDI layer, but it chose to have just one shell.
On the upside, having a consistent GDI library and GUI shell also made it consistent for both developers and users. The corporate world and most average users do not really care if its the same shell or not, they just care that they do not have to keep learning new things to get something done. Windows, like it or not, served that ... and in some sense, overdid themselves. To a large extent, they made it so consistent that its become a drab to use Windows.
All that aside, its undeniable that Windows did make development and usage easier ... not necessarily superior technically, but just easier. My same Windows app from 1997 on Windows95 can run flawlessly on WindowsXP today and I believe will continue to do so even on Windows Vista ... whereas the same app on OZ/OPIE will not necessarily run on OZ/GPE ... much less pdaXrom. Likewise Qtopia apps on other roms. Granted compat lib makes the apps more cross-compatible, but sometimes just sometimes ... you need to repackage or recompile the apps.
Oh, and about unix without gui, there are builds of headless windows (Embedded NT) for certain industrial applications like manufacturing placement machines, control firmware etc. Granted, the Windows we are all accustomed to has a monolithic structure, from MS standpoint, this is what the end user need/want, so its sold as thus.
