This TechNet article offers a great overview of the key security improvements found in Windows 7
TECH NET - An Introduction to Security in Windows 7
http://technet.microsoft.com/en-us/magazine/2009.05.win7.aspx
Table of Contents
* Windows Biometric Framework
* Extending Authentication Protocols
* BitLocker Core Enhancements
* BitLocker To Go
* UAC Improvements
* AppLocker
* Global SACLs and Granular Auditing
* Wrapping Up