This OS X versus Windows XP site contains an exhaustive, extensively illustrated 100-topic comparison of these two operating systems. The author tries to be objective, which is admirable, but the extremely detailed comparison is worth reading mostly because it highlights a lot of subtle design differences. For example, this little feature of the OS X login dialog:
This is a companion discussion topic for the original blog entry at: http://www.codinghorror.com/blog/2005/10/comparing-guis-os-x-vs-windows-xp.html