In the trivia feature on the DVD, it is mentioned that during their fight, Thor breaks the Hulk's jaw, cracks his ribs, and punctures his lung, but the Hulk kept fighting. None of these injuries were mentioned during the movie itself.
Bucking the trend of using celebrity voices, the producers selected the cast they felt could best breathe life into the characters. They wanted the audience to focus on the characters and the story, not spend the film trying to guess who was providing each voice.
In the miniseries Ultimate Comics: Thor, it is revealed that the axe hammer is not restricted by the worthiness test, which explains why Hulk was able to pick it up. However, this contradicts the film's trivia track which states that there is a worthiness test on the hammer.
The filmmakers chose to produce Ultimate Avengers in the traditional 2-D cell animation process as opposed to the current trend of computer generated animation. Producer Craig Kyle explained that he believed that traditional animation remains a powerful medium for conveying character emotion on screen.
In adapting the Ultimates for the screen, the filmmakers retained writer Mark Millar's choice of playing Tony Stark with an air of arrogance, making him come off more as a playboy than an engineering genius.
The original idea was for Captain America to appear during the first Gulf War, not World War II. Writer Mark Millar reconsidered, and kept Captain America's origin as it was in order to provide richer story opportunities.
Iron Man concealing his identity is another departure from the comics. In the comics, Nick Fury wants Tony Stark to join the Avengers because he is a publicly recognized "brand name", and his new Iron Man armor might be useful to S.H.I.E.L.D.
Partially based on the Marvel comic The Ultimates. The plot is a truncated version of the first two story arcs in the Volume 1 series. The characters of Thor, Hulk, and General Fury are watered down versions of the Ultimates comic book characters of the same name. Captain America, Jarvis, Wasp, Giant Man and Iron Man in this cartoon are adapted from the Avengers comic books, as well as Iron Man, Captain America, and Tales to Astonish, published from the 1960s to the early 21st century.