What does Vista SP1 add?

With the release of Windows Vista SP1, Windows Vista is more capable than ever of addressing contemporary business challenges. With a focus on continuous quality improvement, SP1 offers benefits in six key areas:

1. Device coverage:
98 of the top 100 devices are now supported, and more than 47,000 drivers have been added since Windows Vista was released. 99 percent of Windows Vista PCs have drivers for every single  supported device installed.

2. Application compatibility:
Over 200 of the major enterprise applications have been remediated since the release of Windows Vista, representing more than 5 million seats worldwide.

3. Security:
The improved code base of Windows Vista has resulted in less vulnerability over its first year of life compared to Windows XP. In addition to all previously released security updates, Windows Vista SP1 includes security improvements such as enhanced BitLocker Drive Encryption and better support for third-party security and malware protection tools.

4. Reliability:
The continuous, anonymous feedback system built into Windows Vista has allowed Microsoft to address a significant portion of code-related crashes. Early tests indicate that Windows Vista SP1 averages twice as long without disruptions compared to the RTM version.

5. Performance:
Windows Vista SP1 allows faster moving and copying of many files, faster browsing of network shares, and faster recovery from hibernation and sleep states.

6. Manageability:
Terminal Server and Virtualized Desktop modes are simplified in SP1, and expanded Network Diagnostics make it easier for users to diagnose connection problems.

More details here (pdf).