One of the things that really shows is that Gotham City is filled with interesting secondary characters; it's not all the Joker and Two-Face. There are recurring crime bosses like Roland Dagget (voiced by Ed Asner) and Rupert Thorne (voiced by the late John Vernon). They don't have any pathos-infused reason for turning to a life of crime or any signature style or even powers; they're just gangsters who run criminal organizations. Bear in mind that now-standard Batman character Harley Quinn started off as a one-off bit player on Batman: TAS.
Too, what strikes me more and more as I'm watching these episodes is that Batman himself is so often shown in cameo chasing down some penny-ante hoodlum; a purse-snatcher, bank robber, or even coming to the aid of some kids playing "chicken" on the top of a train. You really get the feeling that Batman's life isn't all fighting supervillains and saving the world. There are a lot of "just helping ordinary people" stories in there, too; "P.O.V." shows the same scene from different angles and features the aforementioned Bullock and Montoya, "I Am The Night" is a very introspective piece that introduces yet another memorable, if minor, character; The Jazzman; and in "The Forgotten" we see an amnesiac Bruce Wayne helping some kidnapped drifters and homeless men.
The lessons we can learn from this show, in regards to RPGs, is that even a handful of well-developed secondary or tertiary characters can do amazing things towards making the setting come alive. If the innkeeper doesn't just take the PC's money for ale and a room, but rather starts talking about how the local knights have done in the last four jousts, complete with blow-by-blow accounts of the action, he starts to come alive.
Even if there's only that one "hook", the players will be much more likely to remember him and he becomes more of a fixture rather than just wallpaper. Maybe the quirks and interests of the NPCs will get integrated into the story itself (and so much the better if it happens). If there's to be a joust, and the PCs are thinking of entering, they may just remember that the innkeeper is a keen fan of such things, and go to him for advice on how to defeat the Mauve Knight of Ritterheim.
Also, it points out the value of one-night stand alone adventures. Something that reels in the PCs and diverts them for an evening, but introduces them to a new selection of memorable NPCs and locales (there don't have to be that many as long as they're memorable), will help the setting grow in verisimilitude, which can only help the game as a whole grow in enjoyment for all.