paxx, that you are saying it's a utopia:
Mac & Windows are comercial OSes, that means indepndence for other platforms to be the most used (and purchased OS around). Linux too, gNome and KDE don't are using the same APIS, And BE, and Irix.
And there's the binary compatibility: x86, Itanium, PowerPC, etc. and special processing units (SSE, 3DNow, Altivec).
For this objective, there's the scripting languages, that are working very well on web sites (like Wincustomize) on any computer with a actual web browser.
This idea will be very good, but developpers are not according. There's also the same question for Customization Developpers: Stardock have their different concepts about Lighttek or mss styles. Windows, OSX and KDE-gNome are using the same graphics sources (32 bits colours graphics with alphablends) but are using different kind of files (in my opinion the best option is the gnome/kde, that uses a simple png file).