The biggest addition to this update has to be the supporting of 1080p resolution. People with hi-definition TV's/Monitors will love this! Both games and videos will have the ability to be played back in such a detailed resolution.
This update will also allow the user to stream WMV videos from any Windows PC running Windows Media Player 11 or Media Connect, or from data CDs, DVDs, USB 2.0 flash drives, or Xbox 360 memory cards. Microsoft hopes that the changes will boost interest in the Xbox Live service, especially with Sony's PlayStation 3 just around the corner.
More...Credit: ars technica