Executive Meddling: The Bat-Editors constantly threatened to yank Robin from the book unless the writers obeyed their edicts on how he could be featured; most notably, Robin could not be used in any battle that took place in public, according to the "urban legend" policy in the Bat-books.
This resulted in a couple of Take That! jokes during a reality show story ("Did the all-concealing shadows just say ouch?"), including Robin creating the hero identity of "Mr. Sarcastic," a shaved-head Shout-Out to Spider Jerusalem (and possibly to Protector, a Robin expy created for 80's Teen Titans PSA comics when licensing contracts prevented the Boy Wonder's use).
The Merch: The last few waves of Hasbro's JLA toyline, intended to act as this to Grant Morrison's JLA also acted as this for this series, including figures of Robin, Superboy, Impulse, and Red Tornado.
Beam Me Up, Scotty!: No, Batman did not say "Touch the Batmobile, and you're fired." in "Alienated". While it certainly sounds like something he would say, the line comes from a meme taking the scene where he sees Robin, Nightwing and Batgirl off before leaving out of context. In the actual cartoon, they all sincerely see each other off on good terms.
Channel Hop: Season 3 (Outsiders) aired on the DC Universe streaming service after Young Justice was cancelled in 2013 by Cartoon Network, thus allowing more creative freedom, a consistent release schedule, and a much lesser chance of being Screwed by the Network. This show hops again to HBO Max for Season 4 onwards after DC Universe was discontinued.
Fandom Nod: In the series premiere, you have a citizen asking if Speedy is the Flash's sidekick, and when someone responds that he's Green Arrow's, the first guy says "that makes no sense", echoing a fair amount of comic book fans who didn't get Speedy's name (especially those introduced to him in crossovers).
In Memoriam: The Season 3 episode Evolution which details Vandal Savage's backstory was fittingly dedicated to Savage's original voice actor, Miguel Ferrer.
Lying Creator: Overlapping with Exact Words. Before the second season began airing, Greg Weisman stated that it would continue on minutes after the last season finale ended. While this is technically true, it is very misleading, since the scene that does take place right after the first season lasts less than a minute and is followed by a five year Time Skip.
Artemis's mother used to be a villain named Huntress. She's unrelated to Huntress of the Batman-related DC books. Her comics counterpart was created first (by some decades) and was renamed "Tigress" once the other was introduced.
The Original Darrin: A few of the original actors who weren't in the tie-in game, Legacy, came back for Outsiders, including Alan Tudyk as Psimon, Danny Trejo as Bane, Denise Boutte as Rocket, and Alyson Stoner as Oracle.
Rocket was voiced by Miss Kittienote Kali Troy in "Usual Suspects," "Auld Acquaintance," and "Satisfaction;" Denise Boutte in "Intervention" and "Home Fires;" and Cree Summer in Young Justice: Legacy.
Flash (Barry Allen) was voiced by George Eads in Season 1 and the Season 2 episode "Bloodlines", but was replaced by James Arnold Taylor starting in the season 2 finale. He also replaced Tim Curry as G. Gordon Godfrey as the latter suffered a stroke in the time between Invasion and Outsiders.
On a more minor note, the Tribune in the Rimbor court was voiced by Miguel Ferrer in "War", but he was replaced by Nolan North in the finale.
In Season 3 (Outsiders), Beast Boy is not reprised by Logan Grove, as he left due to enlisting in the US Navy. He's replaced by Greg Cipes, Beast Boy's voice actor from Teen Titans and Teen Titans Go!.
From Gargoyles: Keith David (Goliath and Mongul), Thom Adox (Lexington and Klarion), Bill Fagerbakke (Broadway and Bear), Ed Asner (Hudson and Kent Nelson), Jeff Bennett (Brooklyn and numerous characters such as Alfred and Red Tornado), Marina Sirtis (Demona and Queen Bee), Clancy Brown (Wolf & Hakon and Agent Faraday), Kath Soucie (Princess Katherine & the Weird Sisters and Queen Mera), Brent Spiner (Puck and the Joker), Cree Summer (Hyena and Tula), Gregg Rainwater (Nick & Mr. Maza, and Tye Longshadow), W. Morgan Sheppard (Odin and Sardath), Zehra Fazal (Shari and multiple Young Justice characters including Halo).
Maggie Q was a Wonder Woman fan when she was younger.
Zeno Robinson was ecstatic at being cast as Cyborg.
Reality Subtext: Particularly in Season 3, where issues such as media control, human trafficking, rogue states, dictatorships, and so on are alluded to in some way. Of note is Lex Luthor serving as Secretary-General of the United Nations and recycling rhetoric from Donald Trump, such as "good people on both sides" and "fake news."
Or rather, recycled animation. The opening shot in episode 8 is lifted directly from Batman: Under the Red Hood. The building where The Team fights Clayface is the same one where the mobsters meet in the film.
Red Arrow's apartment is also the same one from the climax of the same film.
Aqualad, Miss Martian, and Superboy guest-starred in a pseudo-crossover with Teen Titans Go! in the episode, "Let's Get Serious". The trio were referred to as Young Justice, Khary Payton reprised his role as Aqualad (and had the only speaking role among the three), and the plot made numerous references to the comparison between the then-cancelled "more serious" show and the widely negatively received Teen Titans Go!. All three characters also appeared in the same designs from this show, though would later receive new designs in line with the Teen Titans Go! artstyle as they made various background cameos (also joined by Artemis) throughout the show as of Teen Titans Go! To the Movies.
From DC Showcase: Green Arrow, Steve Blum and Ariel Winter reprise their respective roles of Count Vertigo and Princess Perdita. Though in the case of the latter, she actually goes for the European accent instead of her regular American accent that she used in the short.
Almost everyone in the main cast reprises their roles here despite the roughly six-year gap between seasons, the exceptions being Beast Boy, Hawkwoman, Lian Nguyen-Harper, Vandal Savage, Ultra-Humanite, and G. Gordon Godfrey.
Greg Cipes reprises Beast Boy in the place of Logan Grove, having played him originally in 2003's Teen Titans and various other adaptations.
From the DC Animated Universe: In addition to Alfred, Emmanuel Jacomy as Superman/Clark Kent, Alexis Tomassian as Robin/Tim Drake, Jean-Claude Sachot as Commissioner Gordon, and the late Marc Alfos as the second voice of Green Lantern/John Stewartnote he replaced Michel Vigné as the character in both parts of the JLU episode The Once and Future Thing.
From The Batman: In addition to Alfred and Gordon, Adrien Antoine as Batman/Bruce Wayne, Chantal Baroin as Batgirl/Barbara Gordon, Edwige Lemoine as Poison Ivy, Thierry Murzeau as Bane, and Christophe Lemoine as the Flash/Barry Allen.
From Batman: The Brave and the Bold: In addition to Batman, Batgirl, Flash/Barry Allen, and Alfred, Donald Reignoux as Nightwing/Dick Grayson AKA Robin Inote most specifically, he voiced the adult version of the character in The Knights of Tomorrow!, while the present-time version was voiced by Alexis Tomassian, Vincent Ropion as the replacement voice of Flash/Barry Allen, Christophe Lemoine as Blue Beetle/Jaime Reyes and the Flash/Jay Garrick, Michel Vigné as Aquaman, Marc Alfos as Red Tornado, Barbara Beretta as Black Canary, Jérôme Pauwels as Captain Marvel, Paolo Domingo as Billy Batson, Philippe Peythieu as Martian Manhunter, Xavier Fagnon as the Joker, and Thierry Murzeau as Black Manta.
Screwed by the Lawyers: Donna Troy was originally supposed to appear in the first season but at the time of development neither Wonder Woman nor her supporting characters were allowed to be in anything in which they werent main characters due to the contract with her creators estate. When Geoff Johns got a promotion in the early 2010s to Chief Creative Officer (the main liaison between DC itself and the various adaptations), he urged DC and WB to renegotiate the deal and they did. The timing didnt work out but they were able to use Cassandra Sandsmark in season 2 as after the time jump, it didnt make sense for Donna to be on a team with a different generation of heroes than her usual team.
Screwed by the Network: For whatever reason the show was very prone to schedule slippage, at least in the US. Cartoon Network took over a year (January 2011 - April 2012) to air the 26 episodes of the first season (not counting a two-episode preview in late 2010). Other countries even finished airing them months before the US did. In season 2, the entire DC Nation block got bumped to January 2013 with no warning near the start of October 2012, when it had only run for two weeks. So that's two back-to-back, three-month-long hiatuses with only two episodes aired in between. This also affected Green Lantern: The Animated Series, and what's worse, none of the creators were told. They all found out from fans on Twitter. Even DC Nation itself wasn't told; they updated their official Facebook asking how viewers enjoyed the block, and ended up having to delete it after it was swarmed by confused and angry fans. The episode was released on iTunes the next day, and peaked at #2 on their sales charts before being pulled, and no one knows why. Both shows were replaced by Beware the Batman and Teen Titans Go! after finishing their seasons. Any explanation for that either? Nope.
Screwed by the Merchandise: Paul Dini revealed on a Kevin Smith podcast that the toyline was indeed the reason for the cancellation. To be more specific, the show failed to sell enough toys. Because of the more detailed animation and the higher budgets it costs, toys have to sell well in order for an action cartoon to stay afloat.
This was likely the reason Outsiders and all subsequent seasons will be streaming-exclusive. This will ensure that no outside execs can screw them over like last time.
Short Run in Peru: Several episodes of season 1 aired in other countries months before their airing in the US. Memorably, because of the infamous October hiatus, Brazilian fans took it upon themselves to record some episodes, subtitle them in English and share them on Youtube until they were shut down.
Kelly Hu voices Cheshire, her mother Paula, and her baby daughter Lian.
Red Arrow and Guardian (both clones of Speedy/Roy Harper, according to Word of God) are both voiced by Crispin Freeman. This is played with in the episode "Salvage", where the now-retired Guardian is one of the people who confronts Red Arrow. And as of "Satisfaction" Speedy finally gets to talk with Red Arrow.
Fred Tatasciore ends up having a fight scene between himself with both characters he voices in episode 10 of Outsiders; Deathstroke and Metamorpho.
Mae Whitman voices both Wonder Girl and her mother, Helena Sandsmark.
Technology Marches On: In "Image" when Superboy, Kid Flash and Robin wonder what "Hello Megan!" is that Miss Martian is referring to, Garfield Logan shows them an old videotape of the sitcom his mother starred in years before. Kid Flash snarks "Whoa, you still have VHS. Where's your 8 track?"
Weisman has suggested that Kaldur isn't straight, though he hasn't directly confirmed it. What Kaldur's sexual orientation would be if not straight is not specified further.
Greg also confirmed that Marie Logan in the show is either a lesbian or bisexual; when asked "is Marie a lesbian or bisexual?", he pointedly answered "Yes". This was also deduced by sharp-eared fans who noted that Queen Bee (who caused Marie's death during the Time Skip) was explicitly stated to only influence most men and some women, which Greg confirmed was indeed based on her victim needing to be sexually attracted to her for her powers to work on them. Unlike with Kaldur however, Greg's answer to the question above suggests that Marie isn't omni- or pan-sexual or another other less common orientation.
When asked whether Bart Allen (Impulse) was gay, Greg Wiesman chose not to answer, saying it was a spoiler. Make of that what you will.
Young Justice'sLoads and Loads of Characters require loads and loads of actors. While they cut costs throughout the series by hiring actors to play multiple characters and not writing dialogue for every character present, they still have way more actors and characters than most similar series. During a panel at Anime Vegas, Greg Weisman and Yuri Lowenthal explained how they got away with it: they took their retake budget and used it to hire more actors. The entire series was done without retakes. To compensate for this, they created what Lowenthal called a "walla library." Basically, they had every character actor get in the booth and record a series of grunts, fight sounds, exclamations, shouts of each other's names, etc. in a variety of different inflections, and stored them just in case they needed it during the sound mix, because they were using the money they'd spend paying the actors to come back and re-record over their mistakes to hire more actors.
This is the first appearance of Miss Martian in DC television.
This is the first appearance of Rocket and Icon in DC television.
This is the first DC animated TV show that features the first animated appearance of Lian Harper, or in this adaptation's case, Lian Nguyen-Harper.
Likewise, this is also the first official appearance of Cassandra Cain in a DC TV show (at the time of her debut in Season 3, she had also already been announced to appear in the then-upcoming Gotham Season 5, and will make her big screen debut in 2020's Birds of Prey), not counting her unnamed cameo in Justice League episode "The Savage Time, Part 1".
Halo previously appeared in Batman: The Brave and the Bold as a voiceless cameo along with fellow Outsider members (including Katana, Black Lightning, Metamorpho, and Geo-Force, with the former three having already appeared in the show) in the Cold Opening to "Requiem for a Scarlet Speedster". And before that, a CrimeSyndicate version of Halo, known as Aurora, appeared in Justice League: Crisis on Two Earths as a member of the Crime Syndicate's version of the Outsiders, the Owlman Faction.
This is the first official appearance of Hardware in a DC TV show. He had been namedropped in Static Shock episode "Gear", where Richie/Gear and Virgil/Static are brainstorming superhero names for the former after he develops superpowers. Richie suggests Hardware, to which Virgil states has already been taking.
This is the first animated appearance of Arrowette in DC television, not counting her silent cameo appearances in the online DC Super Hero Girls cartoon.
This is the second animated appearance of Traci 13. She previously appeared in a cameo role in Teen Titans: The Judas Contract as a civilian that Jaime (Blue Beetle) has a crush on.
This is the first DC TV show to depict Batman along with all five of his current main "sidekicks" (Robin I/Nightwing/Dick Grayson, Batgirl I/Oracle/Barbara Gordon, Robin II/Jason Todd/Red Hood, Robin IV/Spoiler/Stephanie Brown, Batgirl II/Cassandra Cain, in addition to a civilian Bette Kane (if you count her as Batman's sidekick as opposed to Batwoman's) and an infant Damian Wayne).
This is the first official animated appearance of Batwoman (Kate Kane) in a DC television show. She previously made a cameo appearance in Batman vs. Robin as a corpse in a nightmare scene and was a major character in the sequel, Batman: Bad Blood.