Windows 7

1. Up to a 15% performance increase over Vista on some machines especially those that are 64bit capable.
2. Less annoying administration with customizable "consent" confirmation to perform certain tasks.  This does come at a trade off of security, however.  You now have four levels of confirmation rather than just on or off in Vista.
3. Windows 7 Pro, Ultimate, or Enterprise all have solid legacy application support through XP mode.  This mode handles old applications better than Vista's "compatibility" mode.

There are other perks to Windows 7, but it is also important to remember that Microsoft will be fading out XP support and application certification.  Applications may start to not run as reliably and new applications may not run at all on XP.