Yesterday, I purchased a copy of OS X Tiger from my local Apple Store.
I immediately began the install process. I backed everything up, but in my excitement, I instructed the installer to perform an "Update" instead of an "Archive and Install." (Note to self: don't do installs when wife and son are bouncing around the office.)
An "Update" installs Tiger over Panther, leaving a lot of old and unnecessary files laying about the hard disk. "Archive and Install" effectively does a clean install, but creates an archive of your previous system folder in case something goes wrong. You can instruct the installer to "Preserve Home Directories" so you effectively get a clean install and maintain your login ID and files.
The result of my mistake? The 40 GB hard disk on my Powerbook went from having 22 GB of free space to only 10 GB of free space. That's what you get when you don't wipe the disk and do a fresh install.
Looks like that will be next weekend's project...