This is the first DCAU show to start using CGI vehicles. Despite producer Bruce Timm's resistance to CGI, nobody wanted to animate vehicles by hand, and the overseas companies had been begging the production team to switch to 3-D vehicles for years.
Originally, the creative staff didn't think the first twenty-six shows were going to connect in a season-long story arc, until they got to "Justice League Unlimited: Fearful Symmetry (2004)", and realized they already had a story arc in place. They realized that there were all these plot threads that they had going quite a ways in the past from the entire DCAU that could connect into a bigger, broader story arc, and decided to focus all their energy on that.
After the previous Justice League (2001) series, Cartoon Network wanted to re-brand the franchise somehow, and was very vague about it, just wanting to do something to differentiate this show from the original series. Series producer Bruce Timm decided to expand the Justice League.
The creative team literally went through DC's Who's Who books to pick out characters to be used in the expanded Justice League. They made a list and asked DC Comics for permission to use a whole pile of characters at the beginning of the show. They occasionally thought of something new as they went along, and continued to ask on a case-by-case basis. Likewise, occasionally they'd sit on something too long and DC would tell them a character can't be used anymore. Considering the complexity of DC's many licenses, they bent over backwards to allow as many characters to be used as possible.
There were several characters, like Blue Beetle, Firestorm, Black Lightning, the Spectre, and the Phantom Stranger, that were not allowed to appear on the show, due to rights issues making them off limits.
For this series, the scope of Cadmus's vision has expanded somewhat from the cloning facility that it was in the comics into a paramilitary organization dedicated to standing against the perceived threat that the Justice League poses, though the organization's genesis can be traced back to the early days of Batman: The Animated Series (1992).
On this show, Green Lantern John Stewart features a new hairstyle, a bald head, and a goatee. This change is to symbolize that he is trying to move on following the events of the Justice League (2001) series finale "Justice League: Starcrossed: Part III (2004)".
On this show, Shayera Hol (Hawkgirl) no longer wears a mask or any other Thanagarian attire. This is because she is trying to move on following the events of the Justice League (2001) series finale "Justice League: Starcrossed: Part III (2004)". When she rejoined the Justice League, she wore a white and black jumpsuit. She later changed to a yellow and black jumpsuit.
In the final season, Supergirl got a new costume to closer resemble her cousin Superman's costume. In reality, producer Bruce Timm changed Supergirl's costume because he thought her original costume looked dated.
The original inspiration for the title intro for early episodes of this series was the Space: 1999 (1975) main title sequence. The goal was to do a throwback to what old television shows, like Mission: Impossible (1966) and all of the Quinn Martin shows, used to do with "episode-specific" titles. Not too long after its debut, it became a standardized title sequence with regular non-episode-specific clips, because it was extremely difficult to find spectacular, but non-spoiler shots for each twenty-two-minute cartoon. The teaser end titles, which featured clips from the next episode, stayed the same, as per Cartoon Network's request.
Starting early in the second season, producers were restricted from using all Batman-related characters besides Batman himself and a few C-list villains who didn't appear on Batman: The Animated Series (1992). This was because a new unrelated show called The Batman (2004) was set to air on another network, the WB.
Instead of keeping the revamped look of Metropolis from Justice League (2001), the creative team went back to re-using the futuristic deco from Superman: The Animated Series (1996) for the look of Metropolis in this series.
When the Crimson Avenger appeared in the final season, he had a new costume that was slightly different from how he appeared in earlier episodes of the show. The new costume replaced the black business suit and fedora hat, and instead featured a crimson trench coat and crimson hat.
The name "Justice League Unlimited" is actually the name of the future Justice League team seen in Batman Beyond (1999). This series is not about the future team (although they appeared in a few episodes), but instead is an expanded Justice League team put together following the events of the Justice League (2001) series finale.
In this series, a larger Watchtower is unveiled and it is designed to accommodate a large number of support staff and the expanded roster of the Justice League. The Watchtower now has large kitchen, meeting, and medical facilities, and it is strongly implied that some members of the League live on the station full-time.
When the show unexpectedly got picked up for thirteen more episodes, the creative team decided to do an updated version of the Legion of Doom, a group of villains seen on Challenge of the Superfriends (1978). However, DC Comics was totally against having a group of villains called the Legion of Doom, because they felt the name was too dorky (and Solomon Grundy was dead). Instead, the producers brought back Grodd's Secret Society from Justice League (2001), with an expanded roster and a headquarters that resembled the Hall of Doom. However, the later official DVD release of this season by Warner Brothers explicitly calls this expanded Secret Society "The Legion of Doom" on its packaging.
Just like the expanded Justice League, the expanded Secret Society featured one founding member missing. However, unlike Hawkgirl, Clayface never rejoined the team, and is presumed dead after his previous appearance. In reality, Clayface could not be used in the DCAU anymore, because of the Bat-Embargo.
Originally, S.T.R.I.P.E.'s armor was of considerable size, large enough to let Stargirl sit on his shoulder, and to where Pat Dugan had to climb up to get in the armor. However, in the final season, S.T.R.I.P.E. got new armor that appeared to be a perfect size for Pat.
This version of Speedy's design was based on his modern redesign from Teen Titans (2003), but with slight adjustments. This version of Speedy has a different "S" symbol on his uniform, and still uses green colored arrows.
Huntress's original costume was based on the Jim Lee costume, and featured a bright purple cape, shoulder pads, and mask with no white, except for the eye slits. When Huntress finally had a full role, her costume was changed to be an exact copy of the Jim Lee costume.
During the final season, Aquaman and all related characters could no longer be used on the series, due to a television series pilot that was being developed for the WB. Black Manta still got to appear on the show, but has name had to be changed to Devil Ray to avoid rights issues.
Cartoon Network felt that multi-part episodes limited the show into doing big story lines, where so much time and effort has to be put into this one idea and sometimes gets in danger of making stories long, just for length's sake. With this show the network was able to spotlight the characters and give everyone a moment in the sun in one episode, as opposed to doing giant arcs where lots of things happen and there's so much to keep track of. In the bigger sense, multi-part story lines are great, but bad for people who missed a few parts and wonder what's going on when randomly tuning in to an episode. Also, if a network is airing reruns and not doing so in order, there's a risk of someone seeing one part of something and never seeing what happens next or how it all began.
After being used on Justice League (2001), the creative team decided to bring back Doomsday during the Cadmus conspiracy arc, and retrofitted his origin to fit together with the Cadmus conspiracy arc, creating a new backstory that works much better than the centuries-long, confusing arc from the comics.
In this series, Wonder Woman uses her lasso of truth for the first time. Though she had it in Justice League (2001), the lasso didn't have the power to make people tell the truth. In reality, the producers had just decided to omit the ability, because they didn't want to make Wonder Woman seem too powerful.
This series features several characters created by Steve Ditko, including Captain Atom, The Question, Hawk & Dove, The Creeper, and Starman (Prince Gavyn). The creative team wanted to include another Ditko-created character, Blue Beetle (Ted Kord), in the show, especially since they did that episode with his buddy Booster Gold, but the character rights weren't available.
The show had an incredibly successful toyline by Mattel that lasted a few years longer than the animated series. In fact, the toyline was going to be cancelled in 2008 with the Gorilla Grodd 6-pack being the last figures. However, fan support kept the line going as a Target exclusive and exclusive figures sold on Matty Collector until 2011.
In the final season, the Justice League have a new ground base called the Metrotower. It's kind of a cross between the United Nations building and the Hall of Justice from Super Friends (1973). In Batman Beyond (1999) there's actually a Metropolis Tower on the ground. Even though it looks different, it's a later version of the same building. They still have their orbiting Watchtower, though.
The storyline the show did in the first 26 episodes with the Cadmus arc thematically mirrors what happened in "Identity Crisis", a mini-series that DC was publishing at the same time. The synergy between the two stories was not planned, but just a major coincidence. At one point, the show's creative team even talked about Batman breaking off from the League, kind of like he did in the comics in the fallout from "Identity Crisis", but they figured that was taking the story into a direction they didn't want to go.