The first season of Justice League was the shakiest season in Bruce Timm's masterful universe spanning Batman, Superman, Batman Beyond and The Justice League. Superman was a wimp, and the stories were a tad on the dull side. Still, Aquaman was introduces as a tough mutha, and it was great to see non-Superfriends treatments of Wonder Woman, Green Lantern, Hawkgirl, Flash, and any treatment of the Martian Manhunter.
Season 2 changed everything. Starting off with a conflicted Superman taking on Darkseid, the season produced gem after gem culminating in the far too good for TV Hawk invasion of Starcrossed.
After Season 2, the show became Justice League Unlimited and introduced hordes of DC heroes to the mox. No longer were episodes 2 or 3 parters, but action packer 1 parters tied together in a season long arc. Seasons 1 and 2 of JLU (3 and 4 of Justice League) were phenomenal. Superman took on Captain Marvel and Doomsday. The Green Arrow was an incredible new team member. The mysterious Question uncovered a government ploy to "check" the Justice League with its own super research group funded by presidential candidate Lex Luthor. And last, but not least, Batman Beyond got a fitting conclusion that tied the new Batman to all the Justice League Shenanigans.
I won't lie, the last season of JLU was a step down, but nothing could have lived up to one of the most compelling seasons of any show of any genre on TV in season 2. The final season intridcued the Leagion of Doom in a less campy way, and included lots of fanboy shout outs. The Hawkman origin and finale stood out as season greats.
Basically, if you like superhero shows, you will love this show. And even if you don't this is a great series, especially Justice League season 2, and JLU seasons 1&2. Season 1 brings the overall rating down a notch, but an A- aint bad.