I agree with GVBN on everything except /home. In my experience, different distros assign users to different blocks of IDs. For instance, in Mandriva and PCLOS, the first user is ID 500. In Suse, it is ID 1000. Since permissions are assigned based on ID#, not on name, it means a specific user may not be able to even see his home directory!
Another problem I've seen is that not all distros use the same versions of desktops. For instance, one might have KDE 3.5, another might still be using KDE 3.3. There are often significant changes to the initialization files that can screw you up if you use the "wrong" version.
My suggestion would be to keep the /home directories exclusive to each distro. If you need to share data, make a separate "Data" partition, and make it world-read/write, if necessary.