Elimination of Open-source Hardware Support
In order to prevent the creation of hardware emulators of protected output devices, Vista requires a Hardware Functionality Scan (HFS) that can be used to uniquely fingerprint a hardware device to ensure that it's (probably) genuine. In order to do this, the driver on the host PC performs an operation in the hardware (for example rendering 3D content in a graphics card) that produces a result that's unique to that device type.
In order for this to work, the spec requires that the operational details of the device be kept confidential. Obviously anyone who knows enough about the workings of a device to operate it and to write a third-party driver for it (for example one for an open-source OS, or in general just any non-Windows OS) will also know enough to fake the HFS process. The only way to protect the HFS process therefore is to not release any technical details on the device beyond a minimum required for web site reviews and comparison with other products.
This potential “closing” of the PC's historically open platform is an extremely worrying trend. A quarter of a century ago, IBM made the momentous decision to make their PC an open platform by publishing complete hardware details and allowing anyone to compete on the open market. Many small companies, the traditional garage startup, got their start through this. This openness is what created the PC industry, and the reason why most homes (rather than just a few offices, as had been the case until then) have one or more PCs sitting in a corner somewhere. This seems to be a return to the bad old days of 25 years ago when only privileged insiders were able to participate.