Windows XP Features Dropped In Vista
Mary Jo Foley points out that Wikipedia has a list of features that were present in Windows XP, but have been dropped from Windows Vista.

They include:

Windows Messenger (replaced with a link to download Windows Live Messenger).
Netmeeting (replaced by Windows Meeting Space, which people might actually use).
Internet Explorer no longer integrates with Windows Explorer.
MS-CHAP v1 for VPN authentication.
The Pinball game (also, removed links to online MSN games).
Advanced Power Management support.
Support for HTTP email access in the system email client (Windows Live Mail Desktop download required).
IP over 1394
Web Publishing Wizard
HyperTerminal
SerialKeys
Viewing and editing metadata in a file’s Summary tab.