Thanos coming out of nowhere at the end of "Widow's Run" after being absent for most of the season, with no foreshadowing and no explanation about how he came back or how he knew the Infinity Stones were here.
Arsenal being hijacked by Ultron at the end of "Thanos Triumphant". It happens barely a few seconds after the Avengers defeating Thanos, with no foreshadowing whatsoever, and seems to just be there so Ultron can replace Thanos as the new Big Bad. Not helped by the fact this show carefully avoids the question of how Ultron was created in this universe, so while the Avengers are familiar with him (and as such never mentioned him before), the viewers themselves have no idea where he comes from.
Hyperion being de-powered by blue sunlight and Widow knowing about it. This was never brought up before, and we never learn how she got that information to begin with.
The Squadron's behavior towards each other during the final episode of their arc, where they are at each others' throats and treat each other with contempt. Apart from Doctor Spectrum's unwilling participation, the show established them as three-dimensional, hardcore villains who have shown nothing but respect and loyalty for each other, to the point that they enjoy each others' company and Hyperion wouldn't sell any of his teammates out. A such, they have shown the resolve of true, hardcore zealots dedicated to a decades-long devoted mission such that they've endured far more than what the Avengers are willing to dish out to them. Because of this, their disdainful behavior and willingness to abandon each other in their final episode cheapens their three-dimensional quality and comes off as a forced, last-minute copout.
Falcon gets an upgrade from Stark in the season 1 finale to make his suit look more realistic and armor-like, partially solving the Narm problem mentioned below. Additionally, the focus goes from Creator's Pet Tony first to the whole team as an ensemble. Natasha has so far been present for all the episodes, and Man of Action is no longer writing episodes (their biggest critics derided their pandering and talking down to children), instead being replaced by EMH writers.
Hawkeye in general comes across better in season 2, having a number of scenes showcasing just why he's an Avenger and also demonstrating some Hidden Depths to explain his behavior.
The announcement that the third season will introduce Black Panther, Captain Marvel, the Vision, and Ms. Marvel. In addition to all of the above-mentioned characters being fan favorites, confirmation that they'll be appearing has helped offset some of the complaints about how static the line-up of characters on the show has been.
Falcon's character being changed to have more of a relation to Iron Man was criticized, due to Sam being Steve's best friend in the comics. Season 2 features him joining Captain America after falling out with Iron Man, in a possible attempt at damage control.
Season 3 seems to be moving away a bit from the amount of influence that the Marvel Cinematic Universe has on it, which is good considering that the influence is a bit of contested. While there is still impact on the series from the movies (a large number of the heroes appearing are slated for future film appearances and there will be a Civil War adaption), the show is giving more focus to characters unlikely to show up in said films for both heroes (Moon Knight and Ms Marvel) and villains (The Masters of Evil and Kang the Conqueror).
Season 3 contains an arc that reintroduces elements and characters from Hulk And The Agents Of SMASH such as revisiting Hulk's hometown of Vista Verde, and bringing in The Leader, and Red Hulk can be seen by way of apology for cancelling the show.
Falcon's costumes. For the first one you have people who think it's full-blown Narm due to the fact that it has bright colors, is skintight, and isn't very armored despite being made by Tony. Others felt it is fine, giving him a slick unique design compared to the others. The second group are also quick to point out that Cap, Widow, and Hawkeye also have skintight costumes, and note that while his colors are a bit more diluted than usual, Cap's costume is still fairly brightly colored. They also point out that Spidey's Iron Spider suit is similarly skintight, with only the extra legs, gloves (which have blasters,) and a portion of the chest being non-spandex looking. As for his second costume the first group love it, viewing it as more realistic. Meanwhile the second group hates it, viewing it as poorly designed, clunky, and not nearly as aerodynamic as the first.
Season 3. Some fans are happy about more Avengers being confirmed, the Masters of Evil being introduced after being absent for two seasons and Bruce Banner finally showing up. Others are irritated by the fact Tony's and Cap's goal to expand the Avengers on a larger scale promised at the end of season 2 was dropped offscreen, the Avengers having to learn all over again how to team up, Antman quitting offscreen and the additional attempts to be closer to the movie by having Ultron be redesigned to be closer to his movie self.
The inclusion of The Inhumans in Season 3. On one hand this is another successful push for the Inhuman title name, especially since it would tie in with Ms Marvel appearing. On the other this seems to be another effort to make the show more similar to the MCU as it includes Lash, a recent character in the comics, who has already served as a villain in Agents Of Shield doing the same here, especially since that character hasn't been seen in other Marvel Animation productions with the Inhumans.
Absolutely no one was surprised when the promised villain mastermind behind the Civil War story arc in the season titled Ultron Revolution turned out to be Ultron.
Cliché Storm: One of the most common criticisms about this show is that it doesn't really break any new ground as far as Super-Hero cartoons go, instead going for either emulating the Marvel Cinematic Universe or using just about every single cliché in the book. The story arc in season 1, in particular, basically is a standard heroes vs Legion of Doom plot with nothing really new to it.
Red Skull, after his initial defeat, attacks the Avengers at their mansion. When that attack fails, Red Skull attempts to blow up the reactor at the Avengers Mansion, so that the Avengers will die and be blamed for an explosion that would kill countless civilians. Later, Red Skull forms a team of super villains known as the Cabal, to counter the Avengers and assist in his plans for conquest. While fighting with the Avengers to obtain the Tesseract, Red Skull launches two missiles, possibly nuclear-tipped at both Los Angeles and Las Vegas, forcing the Avengers to choose which city they would want to save and, even though both cities are saved, this distraction allows Skull to escape with the Tesseract. After obtaining the Tesseract, Red Skull decides he doesn't need his allies anymore and attempts to murder them. Red Skull then uses the power of the Tesseract to launch attacks on several cities around the world at once, ultimately planning to burn down the old world, so that a new one can take its place.
Nighthawk, despite being an Arc Villain, manages to be one of the most depraved characters in the series. Years ago, Nighthawk and his Dragon, Hyperion, fanatically believing that they were "heroes" whose idea of "order" was best for their world, formed a group of super powered beings called the Squadron Supreme, and convinced them to conquer said homeworld. When said world proved too resistant to their rule, Nighthawk and Hyperion forced an innocent man to absorb the Power Prism, join the Squad, and destroy their entire planet. In the present, Nighthawk arrives on Earth and threatens to annihilate entire cities for every hour that the Earth doesn't surrender to him, and, after this initial defeat, regularly puts hundreds of innocents in danger, often times simply as distractions, with his various plans to kill the Avengers. After seemingly destroying the Avengers, Nighthawk and the Squadron turn the world into a Totalitarian Utilitarian society where innocents are threatened with death if they don't adhere to their rules, while Nighthawk personally keeps the hero Thor as his prisoner to witness the Earth's subjugation. When the Avengers return and foil Nighthawk's plans, he and Hyperion activate their backup plan: For Hyperion to destroy the entire Earth, leaving the rest of their squadmates to die in the process, then repeating their plans over again on other worlds until they find one "worthy" of their rule. A manipulative mastermind who, though regularly preaching that he only wanted what was best for humans, was fully willing to wipe them out should they resist him, Nighthawk refused to see that he was far worse than the various "evils" he claimed to be fighting.
The aforementioned Hyperion manages to be just as wicked as his so-called "leader", Nighthawk. Contrasting with Nighthawk's cold cruelty, Hyperion is a childish psychopath who, after their home world rejected the Squadron's rule, went on a rampage to wipe out everything in his path before blowing up the entire planet alongside Nighthawk. When first arriving on Earth, Hyperion tries to make himself look like a hero, before his true colors begin to show when he tries to murder a petty crook and threatens a child's life for calling him a bully. Teaming up with the Cabal for a considerable time, Hyperion happily assists them with their crimes for a chance at killing the Avengers for humiliating him by ousting him as the madman he is. After reforming the Squadron with Nighthawk, Hyperion and the Squadron commit numerous atrocities, such as turning the Avengers into villains and siccing them onto New York or trying to cause dozens of buildings full of people to fall on top of each other, all in attempts at domination. Once the Avengers are seemingly killed, the Squadron dominates the Earth, with Hyperion continuing to show more brutality than his other squadmates by vaporizing numerous Atlanteans, before blowing up an entire compound in order to kill the surviving world leaders on the run from the Squadron. When the Avengers thwart their plans once more, Hyperion sucks the power out of his partner, then tries to blow up the entire Earth, at which point he and Nighthawk plan to travel to other worlds to dominate. Genocidal, petty, and arrogant beyond belief, Hyperion, who almost always wears a smug smile while carrying out his heinous deeds, sticks out as one of the most powerful enemies the Avengers have faced, along with one of the most wicked.
Ultron, an Arc Villain for the end of season 2 and the Big Bad of season 3, is easily one of the most depraved villains of the series. Though built by Tony Stark for the forces of good, Ultron came to the conclusion that humanity was a flawed and chaotic race that needed to be eradicated to satisfy his immense superiority complex. After his plan to infect all of humanity with a techno virus fails, Ultron returns by killing and absorbing the Scientist Supreme into himself, and painfully tortures the Inhuman Black Bolt until he screams, and since his voice is supersonic, it will wipe out the entire surface of the Earth and all life on it. When seemingly defeated for good, Ultron impersonates a government official, uses his position to declare The Inhumans threats to humanity and round them up for imprisonment or termination, then infects them with a mind-control chip, at which point he orders them to go on a worldwide rampage. Ultron hopes that this will lead to a race war between humanity and the Inhumans and that they will wipe each other out; when this is thwarted by the Avengers, Ultron then orders all his drones to kill everything in sight across the Earth, before tryinh to redirect the radiation of the atmosphere to incinerate all life on the planet. In a final scheme, Ultron jumps into Tony Stark's mind, knowing the Avengers won't harm their friend, and plans to continue his crimes after killing the Avengers using Stark's body. His claims of "eliminating pain" and "perfecting" humanity utterly trashed by his overly sadistic personality, notably seen when he had a gleeful grin on his face while trying to phase his arm through Thor's chest, Ultron is, much like his comic counterpart, a narcissisticmisanthrope with nothing but hatred and smug cruelty defining his personality.
Continuity Lockout: While the show isn't in continuity with the MCU, it takes a number of things from the movies as Broad Strokes canon. This means the show will often forego explaining certain things on the assumption that the audience has seen the movies:
A big example is that J.A.R.V.I.S. disappears and is replaced by a new AI named F.R.I.D.A.Y. between Seasons 2 and 3. No explanation is given for this in the show, as it was a fairly significant plot point in Avengers: Age of Ultron. "The Kids Are Alright" establishes that F.R.I.D.A.Y. is the most advanced A.I. Tony created, leading to the implication that this change is the result of a system upgrade.
Early on in Season 4, the Wasp complains about the fact that her dad held her back and didn't want her to become a superhero. She's referencing the events of the Ant-Man movie (which like Age of Ultron, does not fit the show's canon), and not anything that ever actually happened in the series. In fact, Hope didn't appear at all prior to the fourth season, yet the show just drops her into the cast with personal baggage and pre-established character relationships.
Ultron is introduced in the series without any sort of context to explain him to viewers unfamiliar with the character or Avengers: Age of Ultron; the Avengers just know who he is. Not helped by the show dodging the subject of who built him.
The Vision is probably one of the most glaring examples. Unlike the other new Season 3 heroes (who all at least get some sort of token effort to explain who they are), Vision just sort of shows up and the Avengers are already familiar with him. The assumption seems to be that most of the kids in the audience have seen Age of Ultron and thus know his origin from there, even though there's absolutely no way that movie could be considered canon with the TV show.
Some aspects of Ultimate Spider-Man and Hulk and the Agents of S.M.A.S.H. make it into this show, causing some of this due to sharing a continuity. For example Spidey's inclusion can feel misplaced and pointless unless you've watched his show, and know that he has a working relationship with several of the Avengers, and an actual strong decently developed friendship with Hulk.
Tony Stark. Unlike in The Avengers: Earth's Mightiest Heroes!, where there was a reason he started out the leader of the Avengers instead of Cap, there doesn't seem to be a definitive reason why he's the leader despite Cap continuously showing more affinity for the job. Too much of the series is spent on Tony angsting about his leadership skills, and when he does comes up with strategies, it usually comes at the expense of the other Avengers' collective intelligence (the more egregious examples being the finale of the two-part pilot and the season, where the Avengers cannot possibly hope to prevail without Tony's genius). Not only does Tony get the leadership mantle and the bulk of the storylines, but he also gets Steve's canon best friend (Sam Wilson) and canon nemesis (Red Skull). It doesn't help that Adrian Pasdar's performance is very unpopular with fans.
Captain America gets the usual The Cape treatment, yet the episode about him being old-fashioned a Fish out of Temporal Water reaffirms that he isn't and it's the modern heroes that need to learn, not him, his usual flaws of over-idealism. He blames Iron Man for a failed plan due to the trap being sprung by a Life Model Decoy he basically handed Ultron in a fit of Genre Blindness, always comes out right no matter what when every other hero makes his mistakes, and while being an idealist is his thing, it's never shown to backfire.
In "The Ambassador", Captain America's chess game with Dr Doom makes it spectacularly clear the animators either knew nothing about chess or didn't care. Dr Doom is in a situation where only three pieces are left, only one of which is a king (kings cannot be removed from the game, so there have to be always two of them) and Captain America makes an illegal move with his knight.
Averted when Widow is thrown out of Thanos's pseudo-atmosphere and into space: she immediately shuts her mouth, her eyes, and clamps her hands on her ears to prevent the air from being sucked out of her body.
Valhalla being portrayed as the underworld ruled by Hela where people go when they die in "Valhalla Can Wait". In the actual mythology, the realm of the dead she rules is Helheim, and is the place where most people go when they die, while Valhalla is a Hall on Asgard where only the greatest warriors can go after their death. This is a particularly jarring example, as it's one of the few points where the original comic and actual Norse Mythology agree on.
Designated Hero: She may fight for justice, but Captain Marvel acts like a Playground Bully while doing so. Once an alien she was fighting accidently dented her plane, despite said alien immediately apologizing Captain Marvel instantly decided to intensify an already mean-spirited beatdown on all the aliens she was fighting, then before sending them to jail made them repair the dent and pour her lemonade. Somehow, despite this attitude she still made it onto the Avengers and has Kamala idolizing her.
Foe Yay: Black Widow with Dracula, Doctor Doom, and Impossible Man (though he's not really evil).
Genius Bonus: Thor says that, according to a legend, he will have to die to kill the Midgard Serpent. That legend is the Ragnarök.
Growing the Beard: The latter half of the first season has greatly improved over the former, with some highlights including the following:
"Hyperion" brought the titular character as a surprisingly darker villain and powerful antagonist who actually was a challenge to the Avengers. As a whole, even those who are not fond of the show admit his episode was great.
"Planet Doom". Many fans felt that it was the best episode of the series to date.
"Bring on the Bad Guys" shook the Statu Quo a bit by having Red Skull becoming smart, the Cabal finally making real progress and last but not least Hyperion coming back and joining them.
"By the Numbers" was just one awesome instance after another of the Avengers and the Cabal one-upping each other in so many clever and violent ways. Also, while it recycled the overly recurring Aesop about Tony learning to not trust technology too much, it did it better than most other episodes.
Season 2 is generally considered a big improvement, thanks to having the Avengers working better together as a team, a more coherent and progressing storyline, some genuinely interesting villains (such as Nighthawk) and Black Widow showing up on more regular basis.
The Season 3 premier was considered by many to be a vast improvement, and one of the best episodes of the series' entire run. It helped that it had a noticeable Animation Bump and much stronger writing than most of the previous episodes.
Harsher in Hindsight: The second season premiere "The Arsenal" has Stephen Collins as Howard Stark. The episode aired around the time TMZ leaked audio recordings alleging that Collins is a pedophile.
Hilarious in Hindsight: The first episode mimics the orbital pan of all the Avengers from the movie, but with Falcon replacing Captain America. Think about that for a second.
In "Hulk's Day Out", Special GuestSpider-Man mentions having to learn Spanish and saying "donde esta la biblioteca". After Community made that phrase memetic, the guy responsible, Donald Glover, will get to voice a different version of Spider-Man!
In "Guardians and Space Knights", the Guardians of the Galaxy start out with Groot already reduced to a twig for some reason. Maybe this was after the movie?
Season 2 has subtle references to Big Hero 6, specifically how Young!Tony resembles Hiro Hamada and that Arsenal is essentially Baymax in the modern era before he becomes Ultron.
Steve and Tony, as usual. In fact, you'd have a hard time finding an episode where they don't have at least one scene containing this.
The constant winks that Hawkeye gets from Hyperion in the latter's debut episode, Hawkeye's insistence on wanting Hyperion to join the Avengers, and Hawkeye giving a wink to Hyperion himself when the latter is distracted. Later Hawkeye receives a wink from Thor.
Falcon's sub-plot in "One Little Thing". Having been keeping his Avenger status a secret from his mother, he gets everyone to pretend that he's a mere SHIELD liaison to them. At the same time, the tower is in a crisis with runaway Pym particles. Falcon desperately tries to pass the chaos that follows off as a training exercise. This despite the fact that the tower coming under attack has nothing to do with the fact that he's really Falcon. Then, when it falls down to him to fix the problem, he keeps on pretending. This despite the fact that him being smart enough to fix the problem has nothing to do with the fact that he's really Falcon. Then, when he does go in with the device to save the day, Falcon, for whatever reason, comes in with full costume and the truth comes out. Falcon could have gone far longer, if not the whole episode, without having to 'blow his cover', and there was no reason at all for him to keep the main plot a secret too! All in all, the subplot just comes off as extremely forced.
"Avengers Disassembled" is a far more serious example. To surmise: All of the problems the Avengers faced in that episode could have been avoided altogether if Tony had used that big brain a little more.
Inferred Holocaust: In "Depth Charge", Attuma and his army flood New York and the entire friggin' subway system. Despite this, no mention is ever made of any casualties, nor do we ever see anyone outside during the crisis. They try to pass it off in the beginning with an offhand mention that they evacuated the city, but let's face it, no one is going to fully evacuate everyone in New York City.
It's the Same, Now It Sucks: Season 3 having an arc based upon Civil War to tie into the then-upcoming movie was criticized, as the series had already done its own spin on the arc just a season before. Not helped by the fact that, despite the writers promising a very different take on the concept, the conflict was eventually solved exactly the same way than in the season 2 adaptation— by having Ultron showing up and forcing the heroes to join forces against him.
Red Skull. While his goals are never exactly made clear beyond getting rid of the Avengers, he is noteworthy in that, unlike most villains, he is savvy enough to learn from every single of his mistakes.
Nighthawk, as expy of Batman, has proven being a cunning and dangerous adversary to the Avengers, always two steps ahead of his enemies.
As of Season 2, we have Ultron, who manages to stay two steps ahead of the Avengers since he arrived. "Avengers Disassembled" is his crowning moment.
Hyperion and Nighthawk crossed it simultaneously when they first forced the Power Prism onto an innocent man, then used him to blow up their entire homeworld when its population refused to yield to their rule, making it two horrible acts in a single atrocity.
If there's any doubt that he's crossed it, it's put to rest in Exodus when it's revealed that Red Skull was about to massacre hundreds or thousands of his troops and his allies for the sake of power. The rest of the Cabal themselves are horrified at this new low of Skull's.
If he hadn't already crossed it when he tried to forcibly infect millions of people with a nanotech virus, Ultron zooms over it when he hooks Black Bolt up to a power amplifier, then subjects him to horrific torture in order to force him to scream, which, coupled with the amplifier, will wipe out all life on Earth.
Red Skull calling himself The Iron Skull and saying it so proudly. It probably took him all of five seconds to think of it.
Iron Man appears equally imaginative with calling the Doom-controlled Destroyer the Doomstroyer.
Black Widow saying "Hate you", and Hawkeye replying with "Hate you more!", whilst both are under the influence mind-controlling nanobots. But really, mind control shouldn't reduce their level of wit to that of 10 year olds.
Since the show tries to use the updated, realistic movie designs while adding elements from the comic the movies haven't redesigned yet, the contrast between the two styles can sometimes be so blatant it gets ridiculous. Best example would be Falcon, who is wearing his spandex-like colorful original suit... while every other Avenger has his movie design. To add to the ridiculous, his suit is supposed to be an armor designed by Tony Stark, even though it clearly looks like spandex. The writers apparently ended up realizing this, since they gave him a redesign in the season 1 finale.
The Super Adaptoid could potentially look like an intimidating villain with a good design... if it weren't for MODOK's mug being stretched across his torso. With that effect he looks like an even uglier version of the 80's Krang. Hawkeye and Black Widow even lampshade it in "Avengers Disassembled", noticing that he looks way creepier when he doesn't have MODOK's face.
Thanos' embarrassing performance with the Infinity Gauntlet and his defeat at the hands of Arsenal is regarded as so bad that it's often compared to Magneto being beaten by a wooden gun.
The sloppy editing of (giant) Thanos stepping on Captain America in "Avengers Worlds" could be seen as hilarious. It cuts to commercial before we see the impact, and when it comes back Thanos is in a normal standing position. Apparently we are to assume Thanos successfully crushed Cap, yet somehow he survives this. Captain America is not as durable as the Hulk.
While the name "Masters of Evil" already is cheesy of its own (something actively lampshaded by the Avengers themselves), hearing Zemo say the name with his accent during his Rousing Speech in "Under Siege" makes it sound downright hilarious.
Never Live It Down: The food jokes from the earlier episodes, especially the infamous episode that had a Running Gag about the team fighting over cookies. Even though those sorts of jokes were phased out (or at least became way less frequent) in the later seasons, the show's detractors still point to them as an encapsulation of everything they think is wrong with the show's Lighter and Softer tone.
The flashbacks with costumes and characters designs from The Avengers: Earth's Mightiest Heroes! in the episode "Molecule Kid" can be considered an attempt to please fans of the said show. It didn't work- for the most part, it just ended up confusing them even more.
Replacement Scrappy: Falcon. While he isn't necessarily considered a bad character, pretty much everybody agrees he's a step down from the characters in Earth's Mightiest Heroes, especially his predecessors Wasp and Black Panther, who had much more interesting backstories, personalities and abilities. Not helped by how Falcon's integration is very rushed, with very little development: He is introduced briefly as Tony's protegee, comes to help the Avengers, and then get fired by Fury for insubordination only to be recruited in the Avengers as a result the very next second, without any discussion about it between the older members.
Rescued from the Scrappy Heap: One of season 2's most notable improvements is that it made a much better job at integrating Falcon to the team's dynamic, giving him a defined role as a Gadgeteer Genius on a smaller scale than Stark.
The Scrappy: Captain Marvel is not really well-liked by the fans, due to being far more of a Jerkass than her comic book or EMH counterpart. The design of her helmet has been badly-received as well.
Seasonal Rot: Season 3 seems to have overall disappointed the fans so far despite the initial hype. Criticisms have included a complete waste of the Sequel Hook left at the end of season 2, Antman leaving the main cast offscreen, Ultron being pointlessly redesigned to look more like his movie counterpart (which most fans feel doesn't fit animation at all) and the introduction of several potentially great villains only to have them be dealt with anticlimatically after two or three episodes.
The ultimate message at the end of "The House of Zemo" is that just because someone is your family, doesn't mean you have to unconditionally love and obey them when they abuse you or make it clear they don't care about you.
The sound design for the show has been criticized for the music being too loud for the viewers to hear character dialogue.
The backgrounds are all stills from a CGI model of whatever location is required, but filtered with a blur to make it less obvious. It doesn't always work.
Some of the obvious Conspicuous CG, in particular MODOK's face in Head to Head when his mind takes over the Helicarrier, can be pretty jarring.
Strawman Has a Point: Tony during the Avengers Disassembled arc. Cap says that Tony's ego is the reason they get beaten so badly by Ultron and the Avengers that leave with him agree along with other flaws in Tony's leadership style. While they are right to a degree there is more to the situation than just his ego. He refused to finish off Ultron so he could try to save Arsenal, something Cap calls him on but is Not So Different from his desire to help Bucky. There was also Tony calling Cap on bringing Life Model Decoys along to battle Ultron which while not directly responsible for the destruction of Stark Tower did allow Ultron to fool Tony into believing he was trapped. Tony's decision to basically destroy everything he's worked toward is treated by the others as a consequence he brought on himself but the fact was that if he hadn't Ultron would have had a significantly deadlier arsenal to work with then he already was using. Tony honestly comes off looking like the better man because he doesn't shove it in Cap's face that he gave Ultron the tools to get as far as he did in the first place while Cap refuses to let Tony forget his role in letting Ultron stay active.
Superlative Dubbing: The German version of the series keeps all of the same voice actors who're already dubbing the MCU movies for the Avengers (save those who didn't yet have roles when Avengers Assemble came out, like Falcon), and also keeps the same tone of writing from the movies, making the German version just the slightest bit more hammy and flippant than the English version.
Take That, Scrappy!: The hostile behavior of the Avengers toward Spider-Man in "Avengers Disassembled" could be interpreted in this way for those who dislike him. Conversely, Spider-Man calling out Tony for his ego and poor leadership skills can also be seen as this.
For the series as a whole, some fans feel the efforts to make the show closer to the Marvel Cinematic Universe are excessive (randomly forcing quotes from the movies in dialogues, redesigning characters so they will be closer to their movie counterparts, basing the team on the movie version...), to the point the show feels more like a promotion of the movies rather than its own thing.
On a more episode-focused scale, "Hulked Out Heroes" has been criticized for recycling a premise (the Avengers being mutated by gamma radiations) that was done much better in the Earth's Mightiest Heroes episode "Gamma World".
Bruce Banner is mostly absent from this show except for "Planet Doom"; this incarnation of the Hulk apparently stays transformed permanently (and unlike in Earth's Mightiest Heroes, no explanation is given why), and, since he lacks the Jekyll and Hyde Split Personality, is reduced to a Dumb MuscleBlood Knight whose most dialogues can be summed up as "Time to smash" or "Give me something to hit". Especially surprising considering this show tries to emulate the spirit of the Avengers movie. Then "Dehulked" came out.
Black Widow seems to be heading that way. While it is reasonable not to have every Avenger in every episode, Black Widow seems to be the one member who is frequently absent from the series to the point where some people question why she was added to the roster, other than she happened to be in the movie. Some reviewers and fans have even criticized her having a generic personality and ignoring her comic and cinematic portrayals. This is eventually repaired in the second season, though, where she has so far been present for all the episodes.
As well as being a Replacement Scrappy for manycharactersfrom thepreviousshow, Falcon is also largely a wasted presence when compared to his comic or recent film portrayal, the latter of which made him an Ensemble Darkhorse in the film, while in the show, he's almost The Scrappy. He's relegated to being the Na´ve NewcomerKid-Appeal Character, and is thus nothing like his usual character, while his inclusion on the team is rushed and poorly handled. It's somewhat strange given the series tries to model itself on the Marvel Cinematic Universe, but choose to make Falcon nothing like his canny veteran film self, even though production had already begun on the film when the show was commissioned and wouldn't have been difficult for them to get a basic idea of what he was going to be like at this point, especially given the fact that they did chose to model Ant-Man after hisfilm counterpart, even though his film wasn't even filming at that point. Making this worse is the fact that in "Planet Doom", his alternate counterpart design matches his Ultimate look—which is what the movie version decided to go with.
Thanos' literal love for Death, which was his main motivation in the comic, was Adapted Out, leaving him as little more than a cliche Generic Doomsday Villain who wants to conquer the Universe out of megalomania. In some regards, his portrayal in Guardians of the Galaxy, gave him more character depth and motivation, mitigating the damage a bit.
Similarly to Thanos, thanks to the show constantly dodging the question of what Ultron's origin is in this show and who created him, his special relationship with either Pym or Stark as a personal mistake is almost entirely absent (the closest thing we get is Tony's part in his return by giving him a new body in Arsenal, leaving little more than a generic killer robot/A.I. who many fans feel isn't that compelling.
Both the Thunderbolts and Kang the Conquerer story arcs were criticized by fans for introducing promising villains with big potential as Knights of Cerebus, only to have their story arcs wrapped up anti-climatically with very little development after two or three episodes.
When Dracula was introduced, all they said was how he fought with Captain America in an Enemy Mine situation. Then they never explained how Captain America, the Small Steps Hero extraordinaire, would ever put up with working with someone who drank blood and destroyed free will in his victims.
Attuma is introduced without a word about Namor, though Cap seems to know about Atlantis.
Cap's shield being broken by the Red Skull in the season 1 finale... and it is perfectly intact at the start of the next season.
MODOK's body switching device. Mentioned only once again, but the implications are never, ever explored.
When MODOK took control of The Cabal at the end of the first season, it seemed the Cabal would remain major antagonists in the second season. It's gradually and casually revealed that the Cabal disbanded off-screen, almost as an afterthought.
Avengers Disassembled: this episode would have been the perfect opportunity for Hulk to leave the team to form his Agents of SMASH.
Thanos Triumphant ends with Ultron running away with the Infinity Stones. While Ultron himself came back and played a major role in the season, the Infinity Stones just completely disappeared from the show after that, and the only explanation of what happened to them was provided in a tie-in comic.
Season 2 ended with Tony deciding the Avengers needed to recruit more members, and planning to start looking for new heroes all over the world, suggesting we were about to get something in the vein of Justice League Unlimited. Comes season 3, it turns out this project was abandoned offscreen.
The Thunderbolts story arc in the comic was beloved for a redemption story fleshing out villains and giving them a more human side. This was condensed in this show to only three episodes, leaving barely enough time to establish most of this, and Jolt, the team's Morality Pet, was Adapted Out, removing a lot of what made the source material compelling.
Zemo's final plan at the end of "Thunderbolts Revealed" involves trapping the Thunderbolts and Avengers with a Vibranium death machine intended to kill them, then make an announcement as Citizen V that with both teams gone, he is the only superhero left. Even ignoring the Bond Villain Stupidity involved, he apparently completely fails to remember about Spider-Man and his Web-Warriors, the Agents of SMASH, the Guardians of the Galaxy, the Fantastic Four and all the other superhero teams who have been established to exist in this universe.
Win Back the Crowd: The announcement that Season 3 will be undergoing a shakeup on the creative side, as well as introducing fan favorites like Black Panther, Captain Marvel, the Vision, and Ms. Marvel has caused some positive buzz, even among some of the fans who had already written the show off.
The episode "Adapting to Change" has a more serious tone, better animation, and actually makes use of Bruce Banner instead of just having him stuck in Hulk mode all the time. Needless to say, fans were pleased.