What is known is that there will be some major advancements in compliance and driver support for Longhorn with an addition to securing the OS and making it more central to the home PC. For example:

ADVANCES: Broadcast and video components meet reliability and stability requirements
New built-in driver support is being planned for Windows Longhorn. Support planned at this time includes a new class driver for USB video cameras.
Proposed requirements include the following:
 

• For any device that provides end-user capabilities for transfer of digital content, expose the device schema so that Windows can support seamless content transfer.
• The device and driver must ensure a minimum level of propagation and persistence of device or content metadata.
• The DirectX Video Acceleration (DirectX VA) API must be used if the hardware supports acceleration of MPEG-2 decode with motion compensation, iDCT, or VLD.
• Video that is interlaced must be de-interlaced using the DirectX VA and Direct3D 9 APIs for video.
• The components must be capable of rendering video with the Microsoft DirectShow Video Mixing Renderer using DirectX 9 Direct3D.

You can check out the New Logo Requirements for a bit more technical information about Longhorn.