Not to mention, there's also some pretty big filesystem differences. You may partition them all ext2/ext3, but they all put things in different places.
When I first started playing around with all the distros, one of the main attractions(for me) to Debian was the apt-get package manager. Boy, I thought that was slick. Now, it's been ported out to RPM, and you can use apt-get on Red Hat or Mandrake, and probably a couple others. So the thing that really keeps me there(beyond the want to not start from scratch), is the fact that I know the filesystem inside out. No real want to install a different distro and start searching in /var for things that I'm used to being in /etc