After 2 strong seasons, the Justice League has opened its doors and welcomed what appears to be every other hero in the DC Universe. There are TONS of cameo appearances by classic characters in this episode, but 3 newcomers take center stage:
Supergirl-Virtually unchanged from "Superman:TAS," she's treated as the League Rookie for this episode. Well-written character with a few decent lines to provide comic relief.
Captain Atom: A newcomer to the Animated Universe, this stoic soldier is a welcome addition to the lineup. He's completely devoted to the life of a superhero, and is quite interesting.
Green Arrow: Perhaps the most familiar of the 3, a great new hero. He's the classic "public crusader" from the comics we all know and love. A great touch by the writers was to have BATMAN be the one who convinces Green Arrow to stay, because Batman (more than anyone) knows the importance of someone like Green Arrow.
My only complaint is with the episode's predictability. The ending and events leading up to it can all be seen coming a mile away!