The visual hallmark of the DCAU may have been Bruce Timm's iconic character designs, but there's another thing that brought these shows together: their maturity.
Justice League was no exception to this rule, and while BTAS may be able to boast seminal storylines like Heart of Ice, Feat of Clay and a Bullet for Bullock, Justice League has just as many hits to its name. The team dynamic even allowed the show's producers to push the series in bold new directions, and indeed, in many ways, the show is even smarter for it.
Certainly, Justice League's frank and honest discussions surrounding relationships, trauma and loss deserve to be commended. The series was very adult when it wanted to be (you could probably fill up an article comprised solely of all its innuendos), but it served a purpose in creating a show that catered for all ages.
Another DC series, Young Justice, would later follow in its footsteps, but there's just something uniquely engaging about JL's character-driven ethos. You won't find a series like it.