The major change from Under the Hood was using a completely different explanation for Jason's resurrection. Instead of coming back from the dead via Superboy-Prime Punch, Ra's al Ghul resurrected him with a Lazarus Pit. The Joker now has a larger role, too.
Affably Evil: Ra's al Ghul, who states that he never resorts to violence or killing unless it's for a "greater cause" and is deeply remorseful that his hiring of the Joker was ultimately what lead to Jason's murder; his attempt to fix things via Lazarus Pit resurrection is what sets off the main plot.
As You Know: Used several times throughout the movie to explain who/what many lesser known characters and plot points are.
The Atoner: Ra's al Ghul felt remorseful for indirectly causing Jason's death, via hiring the Joker to keep Batman and Robin distracted, under the delusion he could control him. He attempted to make up for his mistake by stealing Jason's body shortly before it was to be returned to Gotham, and resurrected him using the Lazarus Pit. Unfortunately, this means that Ra's is also responsible for all the trouble Jason causes as the Red Hood.
Audible Sharpness: One of the tetrad of assassins that comes after the Red Hood has glowing blue swords, that look like normal swords when they're not powered up. They also have a loud hum.
Bad Boss: Seriously, how is it even possible that people willingly work for Black Mask?
Compared to his comic-book counterpart, Black Mask is a fantastic employer. He'll beat the crap out of someone every other meeting, sure, but at least he won't personally and horribly torture you For the Evulz or have all his lieutenants slaughtered on a whim.
Badass Boast: Red Hood gets one in his first scene. He stages a meeting between all of Gotham's drug lords, shows up, lays down some demands, and then throws a duffle bag onto the table.
Red Hood: Those are the heads of all of your lieutenants. That took me two hours. Wanna see what I can get done in a whole evening? Make no mistake. I'm not asking you to kick in with me. I'm telling you.
Badass Normal: Batman, obviously, but also every other hero and villain to appear.
Batman Gambit: Despite this being a Batman movie, it's the Red Hood who plays one. Everything the Hood did was to get Black Mask desperate enough to release Joker, so the Hood could provide Batman with his Sadistic Choice.
[A helicopter, which Nightwing and Batman just grappled, starts to crash and fall to the street below]
Nightwing: [referring to the grapple line] Is that gonna hold?
Batman: No. [runs off after the helicopter, grabbing the grapple gun right out of Nightwing's hands]
Nightwing: So you want me to—? Should I—? Okay...I'll just take care of this.
Calling the Old Man Out: The Red Hood/Jason Todd points out Batman isn't as effective at stopping crime as he thinks he is, and by not killing villains like the Joker he allows them to continue hurting people. Yet, it's not like Batman doesn't realize that too.
Came Back Wrong: Played with. While Ra's al Ghul believes Jason Came Back Wrong, going berserk after his dip in the Lazarus Pit, it's later suggested by Jason that it may not be that easy.
Jason Todd: Oh, you got to talking with Ra's, huh? Does it make it easier for you to think my little dip in his fountain of youth turned me rabid? Or is this just the real me?
Canon Foreigner: The techno ninja assassin group, the Fearsome Hand of Four. Their inclusion probably killed two birds with one stone for the creators. First, the original assassin characters from the comic were included to tie Under The Hood with the DCU continuity at the time, something that was dead weight for a stand alone film. And second, with the original characters dropped, they were free to make at least one of them a woman so they could give more balance to a very male cast.
Casting Gag: Given the Ho Yay surrounding Batman and Robin in most interpretations, it's interesting that Nightwing (a former Robin) would be voiced by Neil Patrick Harris of all people.
Chekhov's Skill: Done in reverse. When Batman deploys a line at Red Hood's ankle, Hood is shown cutting the line before it goes taut. Then, in a flashback immediately following that scene, Jason Todd as Robin is shown doing the same move, in what is one of the more obvious clues to the Red Hood's identity.
Chronic Backstabbing Disorder: Black Mask frees Joker from imprisonment so that he can kill Red Hood. Joker promptly tries to publicly burn him and all of his associates alive. But, to be fair to the Joker, his motivation for doing so (beside the fun of it) was to attract the Hood's attention. He's just not the type to think "don't kill the guy who hires you" is an unspoken rule.
Names such as The Penguin, Harvey Dent and the Scarecrow are bandied about by Jason, and he also references "friends [Joker] has crippled". The Joker's quip about the camera could also be a veiled reference to the events of The Killing Joke.
Also, when Batman has the flashback of him cornering the Red Hood, aka. the Joker before he went crazy, at the chemical plant, the Red Hood pleads that he's not a criminal and that he was forced into the job. This could indicate that Joker has the same backstory that he had in The Killing Joke.
The powers that Amazo uses (super strength, flight, laser eyes) were most likely absorbed from Superman.
Black Mask rubs his neck nervously staring at a burning piece of black wood. His face became like it is due to a black wooden mask being burned into it.
Cool Bike: The Batcycle at the beginning of the film, though it is a mite too slow for Batman to get there in time.
Crazy-Prepared: It's a Batman adaptation, so this one is a given, but special mention must be made of Red Hood's helmet. It has a remote-detonated bomb in it. For all those times when he wants to remove his helmet and roll it toward his adversary. He also wears a mask under his helmet.
Curb-Stomp Battle: As soon as Batman disarms the Red Hood near the end, their fight quickly turns into this, with Batman knocking him all over the place. Even more awesome is when you realize that he could have done this at any point, but didn't because he wanted to actually save Jason from himself.
Determinator: Jason in the opening, to an enormous extent. After the Joker's done beating him (inflicting, or so the Joker supposes, broken ribs and a punctured lung), Jason rolls over, gets up, and walks all the way to the front door, slipping and stumbling the entire way, even mustering the energy to try to shake the door open. What really sells it are the close-ups on his bruised and broken face, free of pain or fear and with completely focused eyes. Even when he's facing the bomb ready to explode in seconds, he never loses that composure.
Enhance Button: Being the Batman and having the Batcomputer, this is somewhat justified (Batman and Nightwing automatically enhance a still image of Red Hood). Batman also has an audio enhance button, but we are shown the process of him phasing out the background noise.
Red Hood allows the drug trade to continue if he gets a cut of the cash and the dealers stay away from kids.
Antihero Cannot Comprehend Slightly Lighter Antihero: The main clash in morality comes from Red Hood's belief that Batman doesn't care about him, or the long-term safety of Gotham, because Batman let the Joker live to destroy more people. Batman does care about both, he simply won't break his one rule to prove it.
Evil Counterpart: Red Hood is one to Nightwing. They were both Robin, but Dick works far better with Bruce than Jason did and is not nearly as brutal to criminals. Notable in that Nightwing disappears halfway through the movies, they never meet mano-a-mano. Unsurprising, since Red Hood is driven by his and Batman's past, rather than driven by Nightwing himself.
What Black Mask threatens his newest mook with whilst gawking at his unusual appearance:
Black Mask: You. New guy.
New Guy: Sir!
Black Mask: Don't be nervous, kid. But if you keep staring at me like that... I'm gonna cut your eyes out.
Also, when Jason is resurrected in the Lazarus Pit, he attacks Ra's men in an insane rage. One of whom has the unlucky distinction of having Jason gouging his eyes with his thumbs.
Face Death with Dignity: Jason in the prologue. He sees the timer, knows he can't escape, and his only reaction is a widening of the eyes, before resignation sinks in.
Family-Unfriendly Death: All over the place, though because most of it happens just offscreen, Red Hood setting a guy on fire by shattering a lit Molotov Cocktail in the guy's hand, is unnerving. And The Joker slitting the throat of one of Black Mask's mooks with a broken glass.
Faux Affably Evil: The Joker (who's comedic demeanor thinly disguises the fact that he's a utter psychopath) and Red Hood (snarky and somewhat personable, but this masks some serious issues, as well as an extremely angry, violent persona).
Flamethrower Backfire: Red Hood does this to a mobster by shooting the Molotov Cocktail (petrol bomb) he's about to throw.
Flashback: Batman has several, mostly to Jason's days as Robin.
Flying Brick: The Amazo robot has super-strength, flight and laser eye beams.
Foreshadowing: In a flashback Jason breaks a thug's collarbone when he stops holding back. As the Red Hood he obviously has less compuctions. So in the final fight when Batman himself stops holding back he gives him a beating.
Gender Flip: In the original comic, Ms. Li is a Mr. Li. As the only female character was cut due to Adaptation Distillation, the gender change was likely to avoid having an all-male cast.
Godzilla Threshold: Part of Red Hood's plan was to push Black Mask to this point, so that he would free Joker in an attempt to stop Hood.
Good Guns, Bad Guns: Red Hood has a (heavily modified) AK-47. Talia has a Luger. Both are "bad guy" guns.
Good Running Evil: Red Hood sets this up (for a given definition of "good") by taking over the entire Gotham City drug racket. Partly this is to enforce some standards on the drug selling, and partly to choke out the income of the super-villains profiting by the trade (in this case Black Mask.) He explains that there will always be crime, his modus operandi is to eliminate the worst criminals and keep the others under tight control.
Gory Discretion Shot: Used quite a bit, such as the camera cutting away whenever someone is shot in the head. Also used in the scene where the Red Hood tasers one of the Fearsome Hand assassins in the face, causing his metallic mask to short-circuit and explode, blowing up half of his head and splattering blood on the wall. We are also never shown the contents of Red Hood's bag of the heads of all the mob lieutenants.
I Am the Noun: A drug dealer declares, "I am the east quarter drug trade, you stupid..." before getting interrupted with a gun in the face.
Ironic Echo: At the beginning of the film when the Joker is having a one-sided discussion with Jason, he asks him if the beatings hurt. After the Joker has been captured by the Hood, he gets asked the same question. Both questions are followed by crowbar related violence.
Joker Immunity: Obviously, since the Joker is around. Jason's Face-Heel Turn is mostly motivated by the fact that he's furious that Batman didn't avenge him by killing the Joker.
Jason: Bruce, I forgive you for not saving me. But why? Why on God's earth is he still alive?!
Jumping Off the Slippery Slope: Averted. Jason emphasizes that he's not talking about executing every supercriminal like Riddler or Scarecrow, but makes a very good point asking why Batman wouldn't make one exception to his Thou Shalt Not Kill rule for the mass-murdering Joker. However, Batman points out that killing the Joker would make it that much easier to kill other supercriminals who deserve it less, and based on Jason's methods he'd probably execute guys like Riddler or Scarecrow anyway.
Killed Off Screen: The two guards that helped Black Mask get Joker out of Arkham are revealed in a quick to miss news report that they were murdered in jail while awaiting trial for their involvement.
Knight Templar: Red Hood attempts to restrict the drug market by controlling it, specifically telling the dealers never to sell to kids. His method of enforcing his will on the Gotham underworld often includes murder.
Large Ham: The Joker, but Black Mask stands out by far as the hammiest, screamiest, most hilariously skewed character in the movie - and somehow he manages to be even hammier in the comic the movie is based off of.
Laser Sight: Black Mask has the standard "What's that red dot on my chest... oh" moment when Red Hood attacks his base. Red Hood apparently fitted a laser sight solely in order to provide that moment, since he blatantly waits for Black Mask to notice the red dot before he fires, and the weapon the sight is attached to turns out to be a rocket launcher, which hardly requires pinpoint precision.
Not So Different: Never explicitly mentioned, but this is evident between The Joker and Red Hood. Jason hates the Joker, but as an adult, he is a ruthless killer with a dark, twisted sense of humor, a fixation on Batman, and a dark reputation among Gotham's criminals. To top it all off, he's even using Joker's old nom de crime, Red Hood.
Black Mask saying "Oh, hell," as his reaction to seeing the Red Hood aim a rocket launcher at him.
We also get a nice "Oh crap"-look from the Joker after the Red Hood captures him and the Joker sees him take out a crowbar.
Joker: "So, what's the plan? Slumber Party? Charades? A little Truth or Dare? YES! I'll start with dare!"
[Red Hood pulls out a crowbar]
Joker: "Or maybe I'll just go with truth."
Similarly, Red Hood's "crap" in the final fight, when Batman attacks him with explosive pellets.
Jason Todd gives a sad example in the beginning of the film, where after he crawls to the door of the room where he is being kept only to find it locked, he turns around to see a bomb ready to explode. Jason just gives an apathetic and defeated look that just says, "This Is Gonna Suck, but I accept it."
Pay Evil unto Evil: Red Hood plays this trope HARD. In the beginning, he even went as far as to deliver the heads of mob leaders' highest lieutenants to said mob leaders in a duffel bag to prove a point.
Pensieve Flashback: A couple happen to Batman, including his first meeting with Jason Todd thanks to Jason stealing the wheels from the batmobile and later him, in the Batcave, arguing with Jason as Robin.
Pretty Little Headshots: Taken to Bloodless Carnage levels with some thugs early in the movie. They just suddenly slumped over dead one by one for no readily-apparent-to-the-audience reason, but, luckily, Nightwing is there to quickly exposit that it was a sniper.
Pretty Spry for a Dead Guy: Subverted. Bruce and Alfred do not dispute that Jason died during his encounter with The Joker and dig up his casket. What they discover when they do is that the "Jason" they buried is merely a dummy.
Psycho for Hire: The Joker, in spades. And, once again, he proves that hiring him or doing business with him in general is a verybadidea.
Pyrrhic Victory: Batman manages to stop the Red Hood/Jason and save the Joker's life, but as the Joker points out, though Batman managed to find a way to win, everybody still loses.
Also Black Mask kills all of his remaining lieutenants in an unsuccessful attempt to make a deal with the Hood in the comics.
Start of Darkness: For the Red Hood, it was finding out that Batman hadn't avenged his murder.
And the reason he didn't was because Batman is afraid that killing the Joker would become this for him.
Jason: What? Your moral code just won't allow for that? It's too hard to "cross that line"?
Batman: No! God almighty!... No. It'd be too damned easy. All I have ever wanted to do is kill him. A day doesn't go by I don't think about subjecting him to every horrendous torture he's dealt out to others and then... end him. [...] But if I do that, if I allow myself to go down into that place... I'll never come back.
Take a Fourth Option: When the Red Hood confronts Batman with a Sadistic Choice between killing the Joker himself, letting Jason do it, or shooting Jason to stop him, Bruce manages to block up Jason's gun, causing it to explode in his hand. The Joker is the only one pleased by this.
Technicolor Ninjas: The Fearsome Hand of Four wear green powered armor. Doesn't stop them from sneaking up on Red Hood or giving him a fight for his life.
Fleeing Thug 1: I think I lost him, you think I lost him?
Fleeing Thug 2: I think you lost him... I think we're good.
[Batman immediately lands on the hood of their getaway truck]
Theme Tune Cameo: What sounds an awful lot like the distinctive drum beat of "Molossus" from The Dark Knight Saga plays during the flashback where Batman and Robin pursue the Joker in Sarajevo.
Those Two Guys: Black Mask and Ms. Li. Seriously, they're as different as night and day, and neither of them has a scene without the other. Also, Black Mask lampshades that they're not important to the plot so much as just caught in the crossfire.
Time Bomb: All of the bombs make the compulsory beeping sounds during countdown, allowing the heroes to notice them before they go off. Also, during the endgame when Jason is trying to blow up himself along with everyone else, there is a 20-second countdown rather than an instant trigger to allow Batman to act. Because all bombs need a countdown.
Justified: At this point in time, Jason doesn't really want to kill Batman. Also possibly justified with the Joker (in the intro-flashback) because he's the Joker and would want people to know that they were going to die.
Timeshifted Actor: Jason Todd is played by no less than 3 actors; one as a kid, one as a teen, and one as an adult. The first one is Alex Martella, Vincent Martella's little brother, and he played Robin's teen incarnation! They got to work in the booth together!
Too Dumb to Live: The dealers talking trash to Red Hood when he has not only a gun pointed at them, but the high ground as well. Lampshaded by Red Hood himself.
Dealer: You wanna die? There's easier ways to kill yourself!
Red Hood: Yeah, like yelling at the guy holding the AK-47.
Red Hood's primary motivation for clashing with Batman. The twist is that he isn't pissed at Batman for not saving him, he's pissed at him for not killing the Joker in the wake of his murder. JOKER even gleefully calls Batman out on his sense of priorities. That has to tell you something.
Earlier in a flashback Batman gives one to Jason when he breaks the collarbone of a thug that he was going to interrogate. Jason doesn't apologize for it, but he does apologize for screwing up their mission.
Would Hit a Girl: Red Hood, given his Anti-Hero status, doesn't recognize genders in a fight as he had no problem fighting the katana-wielding female assassin. Same goes for Batman as well.
Wouldn't Hit a Girl: Despite Ms. Li being all snarky at the Black Mask, he never really hits her. He goes directly for the guy beside her. Possibly because, despite how much he yells, he's fully aware that he can't operate without her.