One of the things I hated about clean installing OSX was the previous wallpapers and user avatars weren't kept in the installer. If you were lucky enough to upgrade the OS without errors through each major release you got to hang onto some very important bits of the OS, like Quicktime 7, iPhoto, iMovie. Once you upgraded the hardware, if you didn't have the foresight to backup the application and the app support library folder, you were in for a letdown.
It's the same with iOS too - I've kept the milky way over a snowy mountain top since whatever version it was released because if you change it you lose it (well you have to search around and get it back from the internet).
