There are settings in /usr/share, but nothing I would ever want to mess with. For example, you can reconfigure the behavior of gedit by editing the xml file that controls its operations. That's beyond reasonable customization, if you ask me. Also, a lot of programs store their icons and system images there. If you want to redesign Opera's logo, and replace it across the board (as opposed to just changing the icon), there seems to be a way to do that in /usr/share too.
No /opt in my FC4, so can't comment on that one. Probably similar stuff though - opt is used for non-system programs, which I tend to store in /usr/local. So /opt will have share folders as well.