Originally Posted by
Revolutionary_Jack
The original Batman The Animated Series gets praised or singled out because:
It's the Longest show.
With 85 half-hour episodes (and 2 Made For TV movies - Mask of the Phantasm, Sub-Zero), it's longer than
--Superman TAS (54 half-hour Episodes),
-- The New Batman Adventures (24 half-hour Episodes and 1 Made for TV movie - Mystery of the Batwoman),
-- Justice League (24 half-hour epsidoes in S1, 13 double-part episodes in S2 airing 1hr each. Total both seasons comes about to 52 half-hour episodes, especially once you sub-divide the three-parters and the single half-hour episode 'Comfort and Joy")
-- Justice League Unlimited (39 half-hour episodes)
-- Batman Beyond (52 half-hour episodes + 1 Made for TV Movie - Return of the Joker).
In terms of episodes, JL+JLU combined has BTAS beat but as a single show, BTAS outshines all of them. Since there's so much more of BTAS to dive into than the later series, it does occupy a disproportionate space.
It has a unique visual style
The other DCAU shows have a streamlined visual style, whereas BTAS has a unique aesthetic. It's got Art Deco backgrounds, 30s-50s style car designs and costumes, film noir style visuals and backgrounds. The BTAS show also has those unique title cards that none of the later shows mimicked. For someone interested in aesthetics, the BTAS show is a lot more original and distinct (looking like no other superhero cartoon or for that matter any cartoon in any genre) than the follow-up shows do, which have a greater realism, grounded sensibility, and while quite stylish doesn't stand out so much from other action cartoons made in that time.
It's more standalone whereas later shows are serialized
BTAS isn't continuity heavy at all, except for the 2-Part episodes and one or two instances. You can watch the episodes out of order and any order. It's not serialized. Whereas the later DCAU shows are heavily serialized. Superman TAS for instance builds episode to episode to big arcs dealing with Brainiac and Darkseid. Justice League especially the JLU series is heavily serialized.
It focuses heavily and exclusively on Bruce Wayne/Batman
Most Batman TAS episodes focus on Bruce Wayne/Batman as a character, putting you in his shoes and in his mind. There are big episodes dealing with Robin/Grayson of course but they pale in comparison to the show's focus on Bruce. The later DCAU shows, The New Batman Adventures especially deal with Batfamily stuff, with sidekicks. Batman Beyond deals with his legacy character, Terry McGinnis. Justice League is about an entire ensemble.
Ultimately Kevin Conroy's Batman is the protagonist of the entire DCAU, the entire continuity is about his career from beginning to retirement and old age, so that only adds to the importance of BTAS.
It's a lot less darker than the later shows
This might be a surprise since so many talk about how the Bruce Timm/Dini cartoons are dark and so on. And yeah it was darker in comparison to other cartoons but on the whole the BTAS shows mix dark and light, it has a Batman and Bruce that cracks jokes, smiles, and banters with Alfred, Gordon and others. Whereas the Batman of later series is a jerk and A-hole. If you see it as a whole, you can see it as a tragedy, Bruce slowly becoming darker and colder, and that's addressed in the TNBA episode - "Old Wounds" which shows in flashback how Robin Grayson quit because he couldn't put up with him anymore.
Conclusion
So that focus on a single character and his world, the standalone low-continuity nature of the stories, greater humor, unique aesthetics, and greater volume of content make BTAS the most original, most unique, and most famous of the DCAU. I personally think that's somewhat unfair because the later shows are as good if not better than BTAS. And it's not a case that later stuff hasn't been influential. Like you know Injustice Superman derives from the JL two-parter "A Better World". "Over the Edge" inspired Arkham Knight.