Vista's Reliability and Performance Monitor provides you with insight into how well your system is performing. In previous versions of Windows client (and server), your system collected a variety of information about events related to system reliability and performance, but you had limited ability to visualize that information.
From the Reliability dashboard, I can view the System Stability Chart, which plots a metric of how Vista has been performing over time. This is correlated with relevant system events - such as installs of new applications - that may have impacted reliability. This provides a very useful view of Vista's stability over time that I can use to diagnose and resolve reliability issues.
More information on Reliability and Performance in Vista is here.