The Real Heroes: During the Chitauri invasion of Manhattan, New York's fire department tries to ameliorate the disaster under fire while the NYPD try to fight back even if they only have their side arms. Eventually, the US Army and/or National Guard manage to get troops in for some fire support for the Avengers.
Natalie Portman was enthusiastic about returning to the role of Thor's human love interest Jane Foster, but was heavily pregnant during filming. Jane was thus Put on a Bus to a conference in Tromsų, away from the action of the movie.
Chris Evans quickly grew a full beard once filming was complete, so Cap keeps his face hidden from the camera during The Stinger.
Real Men Love Jesus: Shortly after Thor nabs Loki from the transport, after Black Widow explains that Thor's basically a god, Captain America states that there's only one God in existence, and it's pretty clear that he's referring to the Judeo-Christian God.
Tony: Everything special about you came out of a bottle.
And of course, Tony's speech about how Loki sucks because he provoked six of the most dangerous people in the world and thinks he's going to come out on top.
Reasonable Authority Figure: Nick Fury plays this role. As Tony notes, Nick Fury is a top spy, so everything he says is loaded with half-truths, misdirections, omissions, and good old-fashioned lies, but he has everyone's best interests at heart.
Redemption Rejection: When Thor fights Loki on Stark Tower, he tells him it's not too late to turn back. For a second Loki seems to be considering it. Then he stabs Thor in the stomach.
When the Council sends out jets to nuke Manhattan, overriding his orders, Nick Fury takes things into his own hands. He successfully takes one out with a rocket-propelled grenade. Then the second one takes him by surprise, and he instinctually whips out his sidearm to shoot at it, even though (a) the jet's already out of range, and (b) there's very little chance that a small weapon like that could have done to the jet in the first place. You can practically hear him thinking Damn You, Muscle Memory.
During the battle in New York, Hawkeye finds out that he's out of arrows the hard way: he reaches to pull one out of his quiver and there's nothing there. This startles him so much that he's almost too late to smack a Chitauri warrior in the face when it gets close.
Renegade Russian: The Russian who was interrogating Black Widow was wearing a Russian military uniform and running what is strongly implied to be an illegal arms dealing business.
After the Hulk and Thor work together for a brief to moment, they stand watching something and facing the camera for a second. Cue the Hulk punching Thor out of the shot offhand still staring forward, in revenge for a vicious fight earlier in the movie.
Also invoked in-universe: "Because if we can't protect the Earth, you can be damn well sure we'll avenge it."
Subverted, though. Rogers doesn't let himself get sidetracked by Tony's witty reply, continuing the original discussion about the flaws in Tony's view of heroism.
Rule of Cool: A standard in super hero movies. Hawkeye shooting a Chitauri when his head was facing the other way is a prime example (and something Whedon really liked).
Rule of Three: Phil Coulson seems to have a romantic relationship with a cellist that is mentioned thrice throughout the film. Each time it's mentioned it corresponds to a particular part of the Three Act Structure.
Rust Proof Blood: The blood on Coulson's Captain America cards is still bright red long after it should have dried up. It's our first clue that Fury wasn't telling the truth about where he found them. Note that it smears on the glass table when Steve picks them up.
Same Character, But Different: While more of a freelance hero in the comics, Hawkeye is shown to be an established S.H.I.E.L.D. agent and one of Nick Fury's most trusted men.
Satellite Character: Hawkeye, compared to the other characters, really lacked a dynamic plot arc. He's defined entirely by Black Widow's desire to save him. Prior to The Avengers, his only development was appearing for 5 minutes in Thor (and even then, the average viewer most likely didn't realize who he was in that movie). Come time for this movie, he spends the first two acts under mind control from Loki.
Say My Name: Initially, Tony gets angry and jealous because Pepper calls Coulson by his first name. Later on, when facing Loki in full Mark VI suit, Tony utterly delivers a Pre Ass Kicking One Liner to him:
Tony: And there's someone else you've pissed off. His name was Phil.
Scenery Censor: After his fall to earth, Bruce Banner's nudity is conveniently blocked by debris.
Scenery Gorn: Most of Midtown Manhattan becomes this by the end of the movie.
Schizo Tech: Apart from a few breakthroughs from Stark Industries (and the Tesseract which is Lost Technology), the film is set in a credible early 21st century universe. Even most of the military technology is portrayed as on par with the real world, and globally if you remove the Avengers themselves and the Villains, you mostly get a vanilla modern universe. Mostly. Except S.H.I.E.L.D. is based on a freaking flying aircraft carrier that can make itself invisible. If such technology is available to the Council, why don't they use it more widely?
The entire team, even Captain America. This almost sets the Avengers against S.H.I.E.L.D. in the middle of the film.
The council has made its decision, but given that it's a "stupid-ass decision", Director Fury has merely elected to ignore it, even going so far as to shoot down one of his own birds to curtail it.
Science Hero: Stark and Banner are recruited for this mission specifcally because of their science genuis. The fighting was because they were there to help.
The Scottish Trope: Bruce refuses to call his alter ego Hulk. Instead, he refers to him as The Other Guy. He only slips once, and immediately corrects himself. Though he has made his peace with the Hulk he still doesn't like it. The others don't bother.
Scotty Time: Fury asks Coulson how long it will take to evacuate the research campus and immediately demands that he "do better." Indeed, the Tesseract blows up in less than half that time and many people are killed.
Sequel Escalation: The film involves stakes and action about an order of magnitude higher than any of the previous MCU films.
Sequel Hook: After the main end credits, The Other is shown talking to a person's back, mentioning how fighting the humans after the Chitauri's disastrous defeat would be "to court death". Cue The Reveal from Thanos, smiling. Fans with a keen eye may have noticed the Infinity Gauntlet in Thor. Putting together those two facts will cause this hook to become much more interesting. In addition, Stark Tower now only has an A on it... and Tony seems to be doing some major renovations, like a hangar to hold a Quinjet. If you look closely, he also has plans for a floor for each member of The Avengers.
Serkis Folk: The Incredible Hulk, with Mark Ruffalo himself doing most of the motion capture work.
Shadow Archetype: Loki works as a twisted mirror to the Avengers more than once. He's an example of different parts of their personalities, like Thor's values about becoming king, Black Widow's past murderous life or Tony's big ego, with the incident that led to him becoming Iron Man.
Sharp-Dressed Man: Just like in Thor, whenever he's not wearing his conqueror gear (with the horned helmet), Loki prefers a suave and classy longcoat that would be right at home for a night at the opera.
Loki taunts Nick Fury about coming so close to unlimited power, the very same phrase used by Palpatine when he killed Mace Windu in Star Wars. Both recipients of the speech were played by Samuel L. Jackson.
Tony briefly asks JARVIS if he's ever heard of the story of Jonah, immediately before he goes through the mouth of one of the leviathans and blows it up from the inside out.
Blink-and-you'll miss it, but the image of the iridium that Selvig shows Loki? If you look right at the beginning, the shadows on the iridium sample make it look exactly like Serenity.
The shot during the final battle when Cap, Black Widow and Hawkeye are looking up at the portal watching the first Leviathan come out is shot in the exact same manner in which Peter, Ray, and Egon look at Gozer's portal opening in Ghostbusters(The scene is at 2:19.)
Iron Man's pose during his entrance in Germany: Insane Amounts of bonus points for the circumstances: Captain America is on the ropes and now on the ground without his shield against Loki in both instances, and Hawkeye is using Loki's monologue to his advantage.
Iron Man and Captain America have to work on their ship's engines in flight, with Stark telling Steve to open an access panel. Steve finds himself faced with a mindboggling array of electronics, much as Jayne did while helping Kaylee in the original pilot of Firefly.
Captain America punching the punching bag so hard that it breaks and effortlessly hanging up a new one is taken from a scene in Buffy The Vampire Slayer, switching Buffy for Steve.
That same scene is equally reminiscent of Kingdom Come. Especially considering the nuke was fired by the UN (or similar substitute) with the knowledge it would wipe out the known superhero population. They even have a red-and-gold costumed hero making the sacrifice play to take the nuke out of range, though in this case, Tony survives.
The number of times that characters tell others to "suit up" (even in cases where it doesn't really make sense, like with the Hulk) is probably related to the fact that anyone who recognizes Agent Hill will get the reference.
Tony Stark wanting shawarma, and the team going out to get it in The Stinger, is a shout out to Buffy the Vampire Slayer; one of Whedon's favorite memories from the show was Nick Brendon's audition, after which he asked if anyone wanted to go get shawarma.
Enver Gjokaj appears in a brief scene as a NYPD officer named Saunders.
Another Hulk shout-out apparently occurs earlier, as he chases Black Widow through the Helicarrier, in a nod to the Tank chase in Goldeneye, where a big green monster chases a Russian through a narrow alley, widening it significantly in the process.
Shortly before the movie was made, New York changed license-plate designs, and what you see on the streets is a mix of old and new. Which is what you see in the movie as well.
Selvig's information about iridium is accurate. It is indeed found mostly from meteorites, produces antiprotons, matches its appearance in the film, is one of the rarest elements on Earth, and would be useful for his work with the Tesseract because of its high melting point.
Thor in this incarnation, looking more like the original costume in the comics. He actually gets the sleeves back when things get serious.
Hawkeye's costume is based on his original Ultimates uniform, although his 616 costume is often sleeveless as well.
Smug Super: Loki will remind you at any opportunity that he is a god.
The Smurfette Principle: The casting for the film is even less balanced than the Sixties teams. While the original team had a 4-1 ratio (Hulk left almost as soon as Cap joined) and the second had a 3-1 ratio, the movie's inclusion of Fury and Agent Coulson as "title" characters put the central cast at 7-1. Needless to say, some chunks of fandom took note. Maria Hill was added to adjust the ratio a little.
Snark-to-Snark Combat: Quite a bit of the movie has all the main characters do this to each other. And it's hilarious.
Soft Glass: While Hulk and the Chitauri grunts bash through windows with no problem, and Hawkeye isn't visibly injured by his attempt, though his actor at least has the sense to look like it hurt. Tony also gets thrown out of a window made of fairly thick glass from Stark Tower while not suited up. It doesn't hurt him at all and he seems much more worried about falling to his death.
Fury: I recognize that the Council has made a decision, but given that it's a stupid-ass decision, I've elected to ignore it.
Soundtrack Dissonance: Elegant classical music plays when Loki and Brainwashed!Hawkeye are killing people at a party in Germany. It gives the scene a very surreal feeling.
Inverted during Natasha's chair-bound fight scene. When Agent Coulson calls her, she says, "Let me put you on hold." She then proceeds to fight Russian thugs, accompanied by racing, staccato music, When the scene switches to Coulson listening to the fight, the music continues, sounding like (peculiar) on-hold music.
Speak of the Devil: Banner refuses to call The Hulk by name, referring to him as "the other guy" and correcting himself the one time he does slip in a "he who must not be named" kind of a way.
Spiritual Successor: To multiple past incarnations of Bruce Banner/Hulk. The character is fully re-established in this film and exact details of his past are hardly addressed in a specific nature. The Incredible Hulk has no real bearing on this film, so you could imagine this Banner/Hulk as being the 'older and wiser' version of himself from the 2003 Hulk film or the 2008 film.
The film itself can be considered one to Joss Whedon's run on Astonishing X-Men.
One mid-credits, revealing that the entire Loki-led Chitauri invasion was in fact orchestrated by Thanos.
A second one after the end of the credits, where the crew eats at the shawarma place Tony suggests at the end of the final battle. Whilst it was originally only in the US-release of the film theatrically, it's included in all versions of the home release.
A113 appears in the upper left corner of Fury's video screen as he's having his last videoconference with The Council.
And the Tesseract, a source of infinite power and knowledge with alien origins? Yeah, it's in the S.H.I.E.L.D. books as item #42.
Stout Strength: In a departure from the previous film (and most of his comics, really) The Hulk has a noticeable layer of fat around his massive trunk, though his muscles still show.
Sunglasses at Night: Agent Coulson in his first appearance in the film. He's not just doing it to look cool, though; he has the landing lights of Fury's helicopter in his face, and takes the shades off once he's no longer running the risk of being blinded.
Superhero Packing Heat: Nick Fury and Black Widow. Captain America to a much lesser extent than in his previous film (he uses an assault rifle in one scene). Hawkeye also wears a sidearm, though he prefers using his bow.
Super Toughness: Loki, being a Frost Giant, shrugs off small-arms and assault rifle fire, but is thrown around by explosions. He also understandably gets beaten up by the Hulk, but lives to tell the tale and seems to recover without a hitch by the time he's captured again in the finale.
Supervillain Lair: Loki sets up shop in a warehouse somewhere, loaded to the gills with equipment and mindwiped servants. Oddly, he never seems to return to it after Stuttgart, so there's no scene of the heroes Exploring the Evil Lair.
Tailor-Made Prison: The S.H.I.E.L.D. helicarrier has a cell designed to hold and theoretically kill the Hulk. It's strong enough to stand up to a blow from Thor's hammer with only a crack to show for it, and set up to be dropped from the helicarrier at high altitude if it's damaged in any way. It gets used to hold Loki instead, and Banner doubts its effectiveness on the Hulk when it comes up in conversation.
Tony Stark's favorite tactic. It disgusts Captain America during his "The Reason You Suck" Speech to Stark, because in his mind this means that Stark would never be willing to make a sacrifice if it came down to it.
Fury wants to fight the Chitauri off with the Avengers, while the WSC wants to nuke Manhattan to make sure the aliens are defeated. Iron Man proceeds to grab the nuke after it's been shot, fly through the wormhole and chuck it at the Chitauri fleet, thus managing to both take a third option and make a sacrifice play.
Take That: The album of Alan Silvestri's score (as opposed to the song album — of which exactly one track is in the film). How can a soundtrack be a Take That, you ask? Well, soundtrack fans have long complained about digital releases having extra material not on the CD (such as Silvestri's Captain America: The First Avenger, which doesn't have this on the disc - it was only available as a download). The digital album of the score of The Avengers lasts 64:25... but the physical CD runs 76:17, and several of the tracks on the download last longer on the CD (in particular "Tunnel Chase" — not the first timeSilvestri's used that title — and "Stark Goes Green"), capped with the CD having a whole extra cue ("Interrogation," the music referred to in Soundtrack Dissonance above).
Loki is just warming up to his favourite theme of humanity's inferiority when the Hulk gets bored and just starts flailing him around like a rag doll.
Also applies to a dying Phil Coulson shooting Loki in the middle of a sentence.
Tap on the Head: Black Widow knocks out Hawkeye during their fistfight. Unusual for the trope, it takes two blows to knock him completely out.
Team Dad: Chris Hemsworth sees Tony Stark as "the godfather of the Avengers," which would explain why he's always arguing with The Leader Steve Rogers over what's best for the proverbial kids, like Dr. Banner.
Team Prima Donna: All the Avengers are rather skeptical of the others' abilities and convinced their concerns are the most important ones, but especially Thor and Tony.
The Teaser: Loki stealing the Tesseract from under S.H.I.E.L.D.'s nose.
Terrible Trio: There's Loki, The Other, and Thanos. With Thanos as the leader, The Other as his servant, and Loki as a reluctant ally. The Other serves as communication between the other two, who are each considered antagonists of the film in their own way.
That Came Out Wrong: Agent Coulson is a big fan of Captain America, and watched him while he was sleeping.
The Confrontation: S.H.I.E.L.D. interrogates Loki and they get more than what they bargained for.
The Resolution: The big battle in Manhattan.
In this case it was something of an Enforced Trope. According to Joss Whedon in an article in Wired magazine, Marvel Studios wanted three basic things to happen in the script: a big fight among the Avengers, a set piece in the middle that tore the team apart somehow, and a Big Badass Battle Sequence. Joss goes, "Great, you just gave me your three acts."
Throwing Down the Gauntlet: During a particularly heated argument, Captain America demands Stark put on his armor so they can fight. Tony refuses, however, saying he "isn't afraid to punch an old man" without putting on the suit. note The line "put on the suit" takes on a very different meaning almost immediately after this, when an explosion rocks the entire transport. This time, Tony quickly agrees and puts on the suit.
Title Drop: Unless you saw it in a market which used the title "Avengers Assemble". Double Subverted. While the term "The Avengers" is mentioned casually many times, it's not until Nick Fury and Tony Stark's speeches that the title is given gravitas.
To Be Lawful or Good: A three-way argument to this effect is briefly had between Cap, Stark and Banner.
Token Good Teammate: On a team plagued with detachment from humanity, egotism, anger issues, guilt, and a seething desire for revenge, Captain America stands out because his only flaw is that he's a little old-fashioned. As such, he's the first to realize Loki is playing them against each other, and later on, the team (appropriately) accepts him as The Leader.
Loki antagonizing the Hulk might also count. Even though it might have been a surprise just how quickly he got beaten to a pulp, nobody could have been particularly surprised that it happened. On the other hand, he was probably going to get beaten up anyway and was probably just displaying false bravado in a (doomed) last-ditch attempt to intimidate him.
Tony accuses Agent Coulson of having been this for going up alone against Loki, although Tony seems to be trying to hide his grief over Coulson's death through his trademark snark.
Took a Level in Badass: Loki's first scene, and thus the first scene in the movie, is dedicated to this. He forces his way into a S.H.I.E.L.D. base, wipes out a bunch of agents, mind controls the rest, and steals the Tesseract. This is far greater physical power and ferocity than he demonstrated in Thor.
Black Widow is introduced as a 'helpless' victim about to suffer Cold-Blooded Torture. She is actually drawing information from her would-be torturers, though, and presumably has an escape planned before Agent Coulson intervenes.
In the second act, while other characters are debating the possibility of beating information out of Loki, Black Widow uses real interrogation to deduce Loki's strategy. Loki has also taken the precaution of being ignorant of the MacGuffin's whereabouts so that that information can't be tortured out of him; he fully expects Fury to try it.
The Tower: The newly-built Stark Tower, aptly-if-inadvertently described by Tony (albeit talking about Loki) as "a monument built to the skies, with his name plastered on it". Naturally, it becomes the focus of the Final Battle.
Tragic Keepsake: Fury invoked this by showing Cap and Tony Agent Coulson's bloodied Captain America trading cards, which he had taken out of Coulson's locker and added blood to.
Invoked as a prelude to a superlative ass kicking.
Steve: Doctor Banner? ... Now might be a really good time for you to get angry. Banner: That's my secret, Captain. I'm always angry.
Also the man himself, no pun intended.
Trick Arrow: Though he tends to stick to pointy or exploding arrowheads, Hawkeye has a couple of notable uses of this trope. He fires a computer override arrow at one point, and uses a grappling hook arrow a couple of times. His quiver is the real impressive bit of technology. It stores dozens of specialized arrowheads and automatically attaches new ones to the shafts he's got stored; he can select which arrows he wants at the press of a button. He later takes down a Chitauri hovercraft with a superheating arrow.
It was strongly implied that Black Widow deliberately let herself get captured by the Russian officer early in the film in order to trick him into confessing what he was intending to do before she was forced to abort the operation thanks in part to Loki's arrival.
Loki allowed himself to be captured so he could get his scepter inside the Helicarrier, which supernaturally encouraged the Avengers to start arguing with each other, thus setting up Bruce Banner to go Hulk and smash everything. The spear also allowed Hawkeye to trace the Helicarrier's location so he could disable it and rescue Loki; you can see a readout of the spear on the Quinjet's console.
True Companions: The Avengers are all such big egos (except for the S.H.I.E.L.D. agents) that they can barely be in a room together. They fight and argue, but at the same time, they bring out the best in one another and when they unite with common purpose, they're unstoppable. In short, the Avengers are, as Joss says, "family".
United Nations Is a Superpower: If the "World Security Council" is a Captain Ersatz of the UN Security Council, and S.H.I.E.L.D. is their military arm, they seem to have tremendous authority to override national sovereignty: they can send a U.S.-made fighter plane into Russian airspace with the intent to bomb a building, and launch a nuclear strike on New York.
The Unmasqued World: While everybody had already more-or-less known about Cap (although not his return), Iron Man and the Hulk, by the end of the film mankind has become aware of the existence of extraterrestrials, Asgardians and the Avengers themselves, and have even begun to react much like the mainstream Marvel Universe does with most of their heroes.
Unusual User Interface: The Chitauri gliders are piloted by an elaborate harness that makes it looks like they're steering with their faces.
Unwitting Instigator of Doom: While it's still mostly Loki's fault and he had very good reasons to be suspicious, if Stark had kept quiet on what he figured S.H.I.E.L.D. would do with the Tesseract, things could have gone smoother for the heroes. Banner's more of a pawn.
Unwitting Pawn: Loki's plan to foil S.H.I.E.L.D. hinges on Bruce Banner — not just his condition, but the fact that Banner is smart enough to put half of Loki's clues together before the rest of the Avengers realizes there's a puzzle that needs solving.
Villainous Breakdown: The only reason why the grand schemer Loki would taunt and antagonize the Hulk near the end of the film. Also, the only reason why Loki does anything in this film. With the throne of Asgard out of reach, he apparently wants to be ruler of something, no matter what.
Visible Invisibility: The SHIELD carrier has this, flipped on after it lifts off, but it ceases to be a plot point after that (and certainly didn't help when Loki's posse came knocking).
Loki's rescue team is explicitly shown to be tracking his scepter.
Tony in his Black Sabbath T-shirt; one of Black Sabbath's hits was "Iron Man". The design of the shirt is also a visual pun. The figure on the shirt is a British military pilot with a triangle on his forehead, and the album is called "Never Say Die!" The Iron Man armor started as a flight suit and had a triangular motif (until Whedon changed it) to match the triangular crystal that powered his arc reactor, which is what keeps him alive.
In Black Widow's opening scene, she beats up her captors with a total of eight limbs — her own, and four on the chair.
Wall of Weapons: Seen on board the Helicarrier next to Captain America's shield and new uniform and near a door where Coulson has to do a retina scan to enter. Presumably it's the room where the BFG he uses on Loki is stored.
We Are Struggling Together: The various characters get on each others' nerves when they're off duty, and thus set the stage for Loki's Batman Gambit, but even before they become True Companions, none of them are stupid enough to let personal quarrels stop them from cooperating when trouble starts. This is clearly demonstrated when Cap and Iron Man are on the point of fighting each other, but as soon as the alarm goes off they immediately drop their argument, suit up and have no problem working together in the ensuing chaos. Summed up in the tagline: "Some Assembly Required"
"Well Done, Son" Guy: Stark's still wrestling with his issues, and apparently Howard Stark spent years talking about how awesome the Captain was. And now the Captain is back, like the big brother you can never live up to. All this is confirmed by Downey.
One for the entire MCU. Not only is the first major super team formed, the public now knows about aliens, a huge amount of innocents died in the New York attack, and Thanos has become interested in Earth.
"Phase 2 is S.H.I.E.L.D. uses the Cube to make weapons!"
"To challenge [the humans] is to court Death." Cue Thanos turning around... and grinning.
What Happened to the Mouse?: After the attack on the Helicarrier and the transport of the Tesseract to New York, the remaining rogue scientists and mercs that assisted Loki just up and vanish, leaving him and Selvig to operate and defend the portal device by themselves.
At the start of the film Loki uses Mind Control on Barton, Selvig, and an unnamed SHIELD agent. We see nothing of that SHIELD agent again after Loki's escape.
What the Hell, Hero?: Captain America calls Nick Fury out after discovering that he has been secretly reverse-engineering HYDRA armaments and weapons. He also called him out earlier when Nick Fury arrives to assign him to the Avengers shortly after Loki stole the Tesseract in regards to SHIELD even possessing the tesseract in the first place, simply stating to Fury "You should have left it in the ocean." (referring to Howard Stark finding the Tesseract while searching for Captain America in the previous film).
Why Did It Have to Be Snakes?: The usually fearless Black Widow is terrified of the Hulk until the Final Battle, and not without good cause. This is ironic considering her heavy-handed treatment of Banner earlier in the film.
Why Don't You Just Shoot Him?: Justified in-story: Loki decides to toss Tony out of the window after he fails to take control of him with the spear which was blocked by the arc reactor. Loki could have used his magical ability to trick Tony with an illusion double and attack him from behind with the spear the same way he killed Coulson. Instead he allowed Tony's taunting to make him angry and he lost his temper..
Wicked Cultured: The beautiful opera music that plays as Loki attacks Stuttgart is from Wagner's The Ring of the Nibelung cycle — and more specifically, it is Loge's (and thus Loki's) leitmotif and Loki is obviously very familiar with the music, because he choreographs his attack beautifully with it. Perks to the music editor who snuck that in.
With My Hands Tied: The Black Widow can still kick your ass while tied to a chair, shoeless, and on the phone with Coulson.
The World Is Not Ready: S.H.I.E.L.D. hands the Tesseract over to Asgard at the end, as it makes Earth a target and they clearly aren't ready to possess that kind of power... yet. Also, it's the only way for Thor and Loki to get back to Asgard, where they can build prisons capable of holding him.
World of Badass: It is the Marvel Universe, after all. The film also deconstructs this — it's discussed that while the titular heroes are badass, the rest of the human race outside them and S.H.I.E.L.D. aren't, and the world isn't ready for the new universe suddenly opening up to them, or the threats it brings with it like Loki, the Chitauri, and Thanos.
"World of Cardboard" Speech: Just before the climax, Tony has a conversation with Loki about how he forced their hand to put together this powerful but unstable group. He explained that there is no longer a point where Loki is going to come out unscathed.
Tony: You're missing the point — there's no throne, there is no version of this where you come out on top. Maybe your army comes, and maybe it's too much for us but it's all on you... 'Cause if we can't protect the Earth you can be damn well sure we'll avenge it.
World of Snark: Most of the major characters take a level in sarcasm from their previous portrayals. The most prominent are Tony Stark (as you would expect), Bruce Banner, Loki, and to a lesser extent, Nick Fury. Even the Hulk gets one, after being told by Loki that he stands before a God. His response is to bash him into the ground repeatedly and mutter "Puny god."
Whedon: Everyone in this movie is a dry wit. It's like a desert of wit.
Worthy Opponent: The smile on Thor's face after being punched by the Hulk says it all.
Wounded Gazelle Gambit: Black Widow's interrogation technique uses an interesting form of this. She pretends to be at a disadvantage, prompting her victim to become overconfident and let slip some vital information. While she was playing her reactions to Loki up, she later admits to Hawkeye that he did rattle her pretty badly.
Natalie Portman was unavailable for the film due to pregnancy. Her absence was explained as Jane Foster being transferred to a secure location in an observatory for her protection after Loki shows up.
Hayley Atwell was probably never intended to appear as a (presumably elderly) Peggy Carter, but her character is still referenced on screen by way of a photo (and a deleted scene included on the DVD shows Steve discovering that she is still alive).
Wrestler in All of Us: Black Widow takes out a mook via hurricanrana, Hulk body slams several opponents, and Thor even gets in on the action when he gorilla presses Loki.
During the battle in New York, Hawkeye finds himself on the wrong end of a spear.
When Black Widow tells Captain America and Bruce Banner to enter the interior of the aircraft carrier, Captain America deduced from her comment of it "soon becoming hard to breathe" that the aircraft carrier doubled as a submarine. Turns out it was actually a literal aircraft as well.
Loki, twice. First he mistakenly assumes that Romanoff is going to try to pretend to take his side while he's in captivity. Later, he thinks Tony is going to try to appeal to his humanity.
You Have Failed Me: Not actually done, but promised by The Other should Loki fail to acquire the Tesseract for Thanos, though he was also talking about the possibility of Loki attempting to withhold the Tesseract as well. To wit: "If we don't get the Tesseract, for any reason, we will hurt you in ways you thought impossible and you cannot hide from us."
Our heroes are quite worried that Loki plans on doing this to his mind-controlled minions. Thor worries after Selvig's fate, and Black Widow approaches Loki under the guise of finding out what his plans are for Barton after. His response is not reassuring.
Tony also tries to reason with Loki before the final battle that his employer could very well be planning to execute him once Earth's defenses are breached.
You Just Told Me: This is how Natasha Romanoff/The Black Widow managed to deduce what Loki was planning as his escape. It's also implied earlier that this is how she gets intel, amongother things.
You Wouldn't Like Me When I'm Angry: The Hulk, as expected considering he is the Trope Codifier but subverted in this case.He's always angry and you won't like him when he loses control but you definitely won't like him when he decides to voluntarily unleash the Hulk. Notice how fast and comparatively painless the transformation is before the final battle now that he's accepted the Hulk as part of himself?
Zerg Rush: The aliens, given the kind of opposition they face, rely on sheer numbers to whittle down the heroes. Of particular note is when a good dozen or so of them simultaneously focus fire on The Hulk, effectively pinning him in place under the combined fire.