1. Performance improvements with copying files, hibernation, logging off on domain-joined machines, Javascript parsing in Internet Explorer, network file share browsing, and other areas.

2. Performance improvements with Windows Disk Defragmenter that are also slated for Server 2008. The ability to choose individual drives to defragment is being reintroduced as well.

3. New security APIs for the benefit of antivirus software that currently relies on the unsupported practice of patching the kernel.

4. A new version of Windows Installer, version 4.1.

5. Users will be able to change the default desktop search program to one provided by a third party instead of the Microsoft desktop search program that comes with Windows Vista. Third-party desktop search programs will be able to seamlessly tie in their services into the operating system.

6. Support for the exFAT file system.

7. Support for 802.11n.

8. Terminal Services can connect to an existing session.

9. Pv6 over VPN connections.

10. Support for booting using Extensible Firmware Interface on x64 systems.

11. An update to Direct3D, 10.1,[40] which is expected to make mandatory several features which were previously optional in Direct3D 10 hardware.

12. Support for the Secure Socket Tunneling Protocol.

13. Support for the Group Policy Management Console is being removed; a replacement is planned for release the same time frame as the release of the service pack.

14. Will include a kernel that will be up-to-date with the version to be shipped with Windows Server 2008.

Source