When I studying Classics in college, our mythostructuralism professor, Christian Wolff, taught us about syncretism, the cultural process of reconciling various version of a myth or merging native mythic belief systems. Sometimes, the different versions of a myth coalesce into one larger story, making for an even grander story.
Comic books, being myth, also experience syncretism. In an ancient society, the need for mythic syncretism would arise as the borders of the society expanded to incorporate formerly independent groups. It could be either a friendly process or the result of conquest, and in either case syncretism was used to help ease the transition to a new unity. That yellow-fanged earth mother figure you guys worship, oh, well, that's...umm.. Juno Gingivitis; yes, it's just another... "aspect" of one of our gods. Welcome aboard!
In comic books, the friendly version is the impulse to reconcile different interpretations of a character. Bob Kane's vampire-slaying Batman, the Batman fighting on a giant cash register, Denny O'Neill's James Bond/Sherlock Holmes version, Paul Dini's guardian of Gotham, either of the Frank Miller Batmen--viewing these as one character takes a syncretic mindset. In fact, it's the very basis for what Grant Morrison has been trying to do with his run on Batman (with debatable success).
That syncretic mindset is a little more ingrained in DC readers than in Marvel readers, by the way. It's out of necessity; DC's characters have been intepreted more broadly and with greater variation than Marvel's more tightly on spec characterization of their (originally) coherent literary world.
But the unfriendly version of syncretism is the need to integrate the beliefs of conquered peoples into your own. You don't want to insult the beliefs of anyone involved, but sometimes too many gods can spoil the broth.
And DC certainly has done its fair share of conquering peoples, such as Charlton and Fawcett. Now with increased connections with Wildstorm, the incoporation of the Dakotaverse, and the usurpation of MLJ characters, the DC Universe truly is a multiverse.
How well do you think DC has managed the process of syncretism? They didn't know quite what to do with the Charlton characters after they acquired them. In the beginning, it seems they were being positioned, as ruder and edgier than the DC characters, as the "Marvel" types within the new DCU. Read Crisis on Infinite Earths again, and you won't even recognize Blue Beetle, except by the costume. At some point, this reversed, and the Charlton characters became more lighthearted (in no small part due to their inclusion in the less than grim JLU).
Fawcett, well, the case could easily be made that the DCU never has successfully integrated those characters. Unable to take them at face value, DC has either insisted on making them hokily square or *snort* dark and edgier.
The Wildstorm "integration" has gone much better because, in fact, while the Wildstorm in now clearly part of the large DC multiverse, it is just as clearly "walled off", so that character interactions can take place in a controlled environment. Usually, the problems with characters from different universes interacting isn't that the characters aren't commensurable. After all, the Punisher has met Archie. It's not that the characters can't be made to work together; it's that the worlds they inhabit cannot be the same world. It's not that we can't picture Superman and the Shield together; it's that we can't imagine that a character as amazing as the Shield wouldn't be turning up all over the place in Superman's world.
The literary concept of the shared universe is already a tough one, and the more "worlds" you add to that universe, the tougher it gets. And the job is about to get a lot tougher... .
How well do you think DC has managed the process of syncretism... and how well will it?