Oh! I actually know what made these look so iconic!
The backgrounds for the entire series were done on black paper, a first for any animated show. It's Batman, which means everything is at night and incredibly dark. So, the majority of the backgrounds were going to be black anyways and using black paper would cut down on the time it took to create all of the backgrounds. Adding color over the black paper instead of the traditional method gave it a much more stylized look that helped make it iconic.
It also forced some other creative choices, such as using an airbrush to do all of the background art. During the early testing, they found that using an airbrush was easier to apply paint with than a paintbrush and that it looked better. Airbrushing let them be more experimental with painting techniques, such as the spattery fades you can see in the "Bane" and "Deep Freeze" title cards above or the soft, deeply shadowed face on "the Last Laugh." The overall effect of airbrushing on a black background is a much darker, moodier vibe than could be achieved with a traditional approach.
They cared very deeply about the art of the show and how they were going to create it. It wasn't just about the story or the character. In the words of co-creator Eric Radomski: "As opposed to making shows just to sell toys, we've made quality films."
