According to producer Bruce Timm, this version of John Stewart has been a Green Lantern for so long that the green radiation energy of the ring has infected his bloodstream, so that's why his eyes glow green.
When developing the show, Bruce Timm briefly toyed with the idea of giving the Justice League a uniformed look to their costumes. When Timm mentioned it to DC President Paul Levitz, he said, "Not the big three (Superman, Wonder Woman, and Batman). You can do it to Green Lantern, you can do Flash, but not the big three." The designs were later used as the basis for the Justice Lords costumes.
Was originally to be titled "Justice League of America", after the incredibly popular "Justice League of America" comic (revamped by Grant Morrison). However, "Justice League" was favored, to give the show a more universal feel.
The John Stewart Green Lantern was selected, rather than the better-known Lanterns, Hal Jordan and Kyle Rayner, not only because it would allow for an African-American member of the team, who occasionally appeared in the Justice League comic book, but the producers felt that the character's originally abrasive personality would have more dramatic potential.
The Green Martian characters, such as Martian Manhunter, are voiced by black actresses and black actors, and the Thanagarian characters, such as Hawkgirl, are all voiced by Hispanic actors and actresses.