The main reason so much needs to be rewritten and the reason that new drivers were required on Windows Vista (making it's initial release a fustercluck) is that big media got to Microsoft.
Vista, 7, and 8 have end-to-end DRM encryption support, required for logo compliance with Blu-ray, where the data comes out of the disc encrypted, and goes to the monitor encrypted. The DRM audio comes from Microsoft encrypted, and only comes out of your Zune's headphone jack unencrypted. This fundamentally broke the audio framework, digital audio workstations, video card drivers, imaging devices, etc, because none of these drivers or applications were previously written to prevent users from using data on their own computers.