I remember how I used to buy the old Justice League comicbooks that managed to make it over here to Germany, back in the late 90s I believe. What reason did I buy them for? Because Batman was in there. That's how I got into the whole setting of superheroes getting together and become a team to counter larger threats that one hero alone would not have been able to handle.
And this is sort of where it all goes wrong in my opinion and that's what genuinely turned me away from the animated Justice League. A lot of these episodes are really cheesy if not downright terrible. You know, alien invasions, intergalactic Gods trying to destroy Earth and all that stands in their way is a team of superpowered costumed heroes, it sort of gets old really fast. Not to mention that you'd eventually have to ask yourself "Don't these guys have anything to do in their respective crime-infested cities?"
I remember how I liked those few team-up episodes in the Superman TAS and New Batman Adventures where characters from both shows would cross over. Supergirl would show up in NBA to team up with Batgirl to take down Poison Ivy, Harley Quinn and Livewire, while Batman would come to Metropolis to deal with Joker who teamed up with Lex Luthor to take down Superman etc, as well as Superman having to pose as Batman because the real one went missing. It was nuts, but it was fun. Justice League however, is too much of that.
I don't really know how to describe my gripes with JL/U, but it simply did not really feel right to me. The first series was at least somewhat limited in terms of which characters would appear. You had your original seven members of the Justice League, except for the fact that they threw the "quota black guy" in there in form of John Stewart as the Green Lantern despite the fact that a Green Lantern had already been introduced in the Superman TAS (Kyle Rayner stealing Hal Jordan's origin story). Not to mention that Aquaman is nowhere to be seen except like 2 appearances in Unlimited, getting his spot stolen by Hawkgirl. But then in Unlimited, you've got dozens and dozens of heroes, lots of which I did not even know existed, with cameos all other the place. Sure, there were a few times when I really liked that the spotlight was on heroes other than the main seven, the Huntress/Question episodes in particular were awesome.
Then again, there were really, really stupid episodes. To me, the most ridiculous story in Justice League ever was "The Once and Future Thing". MAJOR [tornado fang]ing sequence breaking all over the place. I mean, they go into the future, Batman meets both his sucessor Terry McGuiness AND his older self and it doesn't even phaze him. How's that supposed to fit with Batman Beyond I'd like to ask. Likewise, that episode were Batman was forced to sing "I'm so blue" on a stage. How can people do that to this kind of character? This isn't The Brave and the Bold you know.
Another thing was the redesigns of the characters. My reaction to the revamp from Batman TAS to New Batman Adventures varied between Yay and Nay (I loved the new design for Batman with just black and gray and hated the lipless Joker), but JL's designs really bugged me. Batman in particular, who seemed to be a fusion of all previous Batman cartoon incarnations. It was basically the same costume design as in NBA with Batman TAS' color scheme and Batman Beyond's elongated cowl-ears. As well as increasing the character's overall muscular structure. It looked terrible in my opinion. I also did not like Huntress' design, but apparently as the show was made, that was her current design in the comics whilst I knew her for wearing that fullbody suit with the cross around her neck from the 90s (The Plague etc, you know, when Azrael was still alive).
Justice League/Unlimited has both good and bad points, but as things are for me, DC never really managed to do something really awesome again after Batman TAS, and I'm saying that with my nostalgia glasses put off.