Justice League Unlimited one of the most ambitious American cartoons ever produced. JLU is really the culmination of 15 years of the Timmverse.
The Timmverse is the DC Comics animated universe created by Bruce Timm and his partners. It started in the early 90s with Batman: The Animated Series, added Superman: The Animated Series, and ended with JLU.
Batman: TAS started out very self-contained and viewers saw little of the larger DC universe. Superman: TAS opened it up somewhat. But JLU just exploded onto the screen. Dozens and dozens of DC heroes and villains were sharing the screen and sharing adventures. Some of these heroes were amazingly obscure for a mainstream animated project.
It's really amazing what was accomplished here. Many of these heroes and villains had never before been animated or on TV. And many might never be again. It was possibly a once in a lifetime event for fans of the DC Universe. Thanks you Mr. Timm!