Justice League was a great series to me for a lot of reasons. I enjoyed the amount of character interaction, I also enjoy how well the continuity of all the superheroes blends. It doesn't feel like the characters were plucked out of their environment and thrust into a storyline of which they had no interest. Of the seven heroes, the writers picked some of the very best. While Batman and Superman are pretty obligatory choices, John Stewart (AKA The Green Lantern) was a choice that I rather enjoyed, him being my favorite GL and all. John Jonzz and Hawk Girl were also very interesting choices to me, as they are on the more obscure side of the DC comics.
Characters aside, the animation is just fantastic. Not to say it doesn't have it's small blips, but for what little cheesiness, there is so much more good.
No expense is paid when it comes to the bad guys either. You want Luthor, you got him; you want Darkseid, you got him, Braniac, Savage, and even The Joker. I even enjoyed seeing Gorilla Grodd make an appearance, way cool.
While this show was very well done, it did have some drawbacks (to me at least). One of the things I didn't enjoy was the episode style. Justice League seemed more like a compilation of seperate story, as just about every single story was a two(or three)-parter, with the exception of the christmas episode. Another thing I didn't enjoy were some of the story ends. Characters like Amazo and Aquaman never really got legitmate endings to their storyline arcs, either because no one could defeat them, or because no one could give them what they need, seemed like a cop out to me.
For what it's worth, Justice League was a really really good show, and if you enjoy comics as much as I do, you might really enjoy this series!