In previous versions of Windows, there were special views you could set as the default, such as the picture folder type that had a much better viewer on the sidebar with a bigger thumbnail and could even display animated gifs. It was also possible in Win2k to have a media player in the sidebar for audio/video files.
When XP came along, you lost the ability to have these custom folder templates, and you can no longer edit an existing one or even make your own.
One of the things I really miss from WinME is the pie charts on the sidebar when at the root of a drive. In XP I have to go to My Computer and right click a drive and view the properties to see it. I think having it automatically showing in the Explorer sidebar made me much more aware of how much space I was actually using or had left.