The term "public domain" is very misleading. When it is tossed about, I think people are usually referring to copyrightable works and it is true that copyrights do expire. If you want to figure out when a particular copyrighted work expires....have fun. It is about as clear as mud, but there are flowcharts out there. This utility on the Rutgers website is useful:
http://comminfo.rutgers.edu/~lesk/copyrenew.html
BUT....the other thing to remember is that trademarks can be renewed in perpetuity forever as long as the owner of the mark is still using it.
So, even though some old issues of Batman will someday exhaust their copyright protections, all that will mean is that you or I could reproduce an exact copy of the Batman issue and sell it. Kinda like anyone can print and sell copies of Moby Dick.
It doesn't mean that we can start creating new stories featuring Batman, Bruce Wayne, Gotham, Robin, the various Bat-symbols and all other manner of DC-owned trademarks (both registered and unregistered). Those trademarks will endure for as long as anyone cares about comics.
So, the best examples of things that anyone can play with are going to be characters from mythology and fairy tales.
I don't really understand the attraction to characters/stories in the public domain. Even if you can create a great story about Zeus, why not use that same story with a newly created character that you can protect?