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 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.
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.
The sequence of Captain America risking his life to stop a bomb is taken right from the pages of The Ultimates #1.
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.