Originally the director had intended for Wonder Woman to battle Solomon Grundy in the intro movie, but opted for Metallo to avoid issues with the ESRB. Cutting off his arm and pulling the sword from his back to chop off his head made it in the final cut by arguing "he's just a robot."
DCUO features the very last appearance by Mark Hamill as The Joker. A lot of people think it was Arkham City, but he recorded VO for our fourth DLC Pack, The Last Laugh, well after Arkham City had released.
Jim Lee had established a relationship with Sony Online's developers years before they were involved with DCUO because he was a diehard fan of EverQuest. In fact, Jim Lee was a hardcore EverQuest player back in the day -- he was the first on his server to obtain the epic weapon, Fiery Avenger.