Iron Fist: Finn Jones, Jessica Henwick, David Wenham, Jessica Stroup and Tom Pelphrey.
Awesome, Dear Boy: Glenn Close was ecstatic to play Nova Prime in Guardians of the Galaxy, having always wanted to play an M-like authority figure. By all accounts she enjoyed filming the movie and immediately said she was eager to return for a second.
Cash Cow Franchise: Four of the films have made over a billion dollars in ticket sales alone. Most franchises are lucky to get one movie that makes that much. As of Avengers: Age of Ultron, it has taken Harry Potter's spot as the highest grossing film franchise of all time.
Colbert Bump: One of the benefits of not having Spider-Man and the X-Men available to use is that Marvel put their B- and C-list heroes in the spotlight instead, causing the likes of the Hulk and Captain America to regain the mainstream notoriety they once had, saving Iron Man from slipping into C-List obscurity, and giving the Guardians of the Galaxy the mainstream appeal they had never had before. The Guardians in particular went from struggling to keep a single comic on the shelves to having six solo or team books by the following year, their own cartoon, and more merchandise sales than they'd ever seen before. This more or less came full circle when Spider-Man joined the MCU with Captain America: Civil War.
Joss Whedon has expressed some regret that Marvel's insistence that he helm Age of Ultron prevented him from working on Agents of S.H.I.E.L.D.
Actor Terrence Howard was not happy at how he was dropped from the franchise. He claims that his contract stated he would be paid $8 million for Iron Man 2, but the studio told him they would only pay him a fraction of that, and he could either take it or leave. He left and James Rhodes has been played by Don Cheadle ever since. He also claims that most of the money in his contract went to Robert Downey Jr.'s salary.
Jeremy Renner wasn't happy with his Brainwashed and CrazyAvengers role that kept him out of the team dynamic for most of the first film, saying it "wasn't what I signed on for"; Whedon placated him with a much beefed up central role in Age of Ultron. He also has a very team-based role in Civil War.
Chloe Bennet has become very fed up with how little contact Agents of SHIELD and the other TV shows have with the movies. It's been implied that this is also the case with quite a few other people involved with the shows, though she's so far the only one who's dared to say so in public.
Darkhorse Casting: To an extent. For the most part Marvel Studios prefers to cast more obscure actors or have them Playing Against Type as the film leads, while relegating more famous actors to side characters. As a result the films tend to be a Star-Making Role for them.
Dawson Casting: In a weird way. The new actor for Spider-Man in the Marvel Cinematic Universe is 19 year old Tom Holland, who people claim "looks too young" to play 15-year old Peter Parker. Given that Tobey MacGuire and Andrew Garfield were both in their late 20s when they first started their own movie franchises - a Peter who was a late teen, at best - this is particularly absurd logic.
While not an extreme case, Hayley Atwell is older than Peggy Carter in the 40s-based scenes - she was 28 when filming a Peggy who aged from her early-to-mid 20s, and plays a late-20s version when Atwell herself is in her early-30s. In the same context, Dominic Cooper plays Howard Stark aged in his mid-to-late 20s when Cooper himself was in his early-to-mid 30s age range. Of course, Atwell also plays Peggy at much older than her real age in Winter Soldier and Ant Man, while the older Howard is played by John Slattery.
Development Hell: Before ever getting its own studio, Marvel had plans to produce films of Iron Man, Thor, Ant Man, Black Panther, Iron Fist, and Luke Cage. Now, this is a reality (although Iron Fist and Luke Cage will be television properties).
Ike Perlmutter and a select group of cronies kept interfering with Kevin Feige and the film properties. The stress this caused played a large part in people like Jon Favreau and Joss Whedon declining to return to the franchise. After Disney purchased Marvel, Feige invoked a higher level of meddling, threatening to walk unless Perlmutter was cut off from Marvel Studios entirely. Disney obliged by restructuring Marvel so that Marvel Studios became a separate entity and Feige didn't have to answer to Perlmutter anymore. This caused so much relief that Chris Evans even said that he might consider extending his Marvel contract (he'd previously been adamant that he'd leave once it was over, due to Perlmutter) and Favreau and Whedon have even said that they wouldn't rule out returning to direct more Marvel films.
In addition, Perlmutter is rumored to have kept Feige from pursuing film projects based on Black Panther and Captain Marvel (and is strongly implied to be the key reason why there hasn't been a Black Widow film, since he's vetoed merchandising the character and was revealed to have been against female-led superhero movies in general). Perlmutter is said to have agreed to let Feige make those movies on the condition that an Inhumans movie was greenlit for Phase 3 (as Perlmutter had actively been trying to build up the Inhumans property to serve as an alternative to the X-Men series), even though introducing the cast of characters at that point would probably not be the best idea, given that Avengers: Infinity War was meant to wrap up many of the major story arcs that had built up to that point. Since Feige was given more power, the Inhumans movie was taken off of the Phase 3 slate and saved for a later time.
Marvel's executives cajoled Joss Whedon into including an odd, unnecessary scene in  where Thor took a dip in what can only be described as a prophecy pool in order to set up ''Thor: Ragnarok."
More generally, Marvel has occasionally been criticized for imposing a "house style" on its movies that make all of the films look and feel basically similar. This level of control means that the MCU maintains a consistent tone and there are few deep drops in quality, but it also means that the unique voices of writers and directors can be sublimated to the universe's franchise demands.
Due to rights issues, the Spider-Man, Fantastic Four, Sub-Mariner, and X-Men characters could not appear in Marvel Cinematic Universe at the time it was created. So, while we can't see the Hulk fight Juggernaut anytime soon, there are gray areas. Some characters who are known to these groups, such as Quicksilver, Scarlet Witch, the Skrulls, and The Kingpin, are still usable, but they cannot mention their connection to those groups.
Spider-Man was integrated into the setting before Phase 3 began. However, due to legal restrictions from Sony's contract, along with the tensions between producers Ike Perlmutter (who leads Marvel's television division) and Kevin Feige (who leads Marvel's in-house movie division), Spider-Man characters are currently unable to be used in any of the shows set in the MCU.
Per interviews given about Agents of S.H.I.E.L.D., it's now confirmed that the MCU cannot use the word "Mutant", so they often use other nomenclature, such as "Gifted", for the mutant-like beings that show up on that show. Whether this is because of some sort of written agreement or just an over-abundance of caution about possible legal issues (as mutants and mutation are actual scientific concepts and are more-or-less generic terms) with FOX is unknown. Director James Gunn has confirmed that a similar joint rights agreement exists for the Skrulls, who are a general Marvel Universe threat but nonetheless debuted in the Fantastic Four.note The agreement is that the race can be used, but not certain Skrull characters tied explicitly to the Fantastic Four or Silver Surfer properties—namely, Super-Skrull (because he has all of the FF's powers). The alien race known as the Badoon are owned by Fox and off-limits to Marvel, which led to them being barred from appearing in the Guardians of the Galaxy movie (but not from making a cameo in one of the prelude comics).
The Hulk is also in a form of exile, as while Marvel has development rights to make movies with the Hulk, Universal retains distribution rights to get them into theaters. Hulk characters can show up in ensemble movies no problem, but there's no incentive for Marvel to make a Hulk standalone movie if they have to split the profits with someone else. In addition to Hulk, the Sub-Mariner property is also in a strange situation with Universal. They have owned the the film rights to the character of Namor for years, though they have not used him; however, they are supposedly interested in sharing the property with Disney provided that they can profit off of it. This is all in spite of the fact that the legal rights on a Sub-Mariner movie should have long since expired.
Like Sub-Mariner, Man-Thing is in a similar position, where he is owned by Lionsgate, but the company isn't interested in making another solo film with the character unless they can reap a portion of the rewards. This got more complicated when Man-Thing was mentioned in an episode of Agents of S.H.I.E.L.D., and given that no statement has been made regarding the character, it's completely ambiguous as to who owns the property (although, given the lack of any comment from Lionsgate regarding a reboot or a sequel, it can be inferred that the rights quietly reverted to Marvel).
A self-imposed example can be found in the form of Ant-Man; though Marvel owned the film rights to the character, Edgar Wright didn't want the company to do anything with him until he made a movie starring the character. After the Development Hell passed, the characters in that series are free to show up in the MCU proper.
Follow the Leader: Before 2008 and Iron Man, crossover films were thought to be a novelty, and the idea of a series of separate films with different writers and directors sharing a continuity and ongoing story was seen as impossible. Now, everyone wants to repeat the success of the Marvel Cinematic Universe and try to create their own franchise of inter-connected blockbusters:
Universal is rebooting Universal Horror into a series of action-adventure films starting with Dracula Untold and continuing with a re-imagining of The Mummy. While not superheroic in nature, the movies treat their major characters as if they were superheroes.
God Never Said That: While doing press for Age of Ultron, director Joss Whedon was asked about Phil Coulson's resurrection on Agents of SHIELD. Whedon explained he thought bringing it up would take away from the emotional impact of Coulson's death, and that Coulson was still dead (to the Avengers). While what he meant was clear from the context, it immediately exploded across the internet, even on semi-respectable pop culture sites, into him declaring Agents of SHIELD (which he executives produces and his brother and sister-in-law are showrunning) as non-canon and a product of corporate infighting and, event more tenuously, that he and Marvel hated the show and wanted to kill it as soon as possible.
Paul Bettany has stated that he sees his role as JARVIS as simply a source of easy money and has never even seen the Iron Man films. However, he's been much more invested in his new role as The Vision.
Bettany: "I used to be in a studio for 45 minutes and do J.A.R.V.I.S. and get a huge bag of cash and go my way like a burglar, and now they want me to work for my money. Which is great and sweaty and hot, which youíll realize once they unveil everything. Itís really f***ing cool. Itís great to join this train which is on really clear tracks."
Clark Gregg says this was initially the case. He took the role of Agent Coulson only as a favour to Jon Favreau, his neighbor at the time. However he ended up enjoying the role more than he expected, leading to Coulson becoming a Breakout Character.
Despite signing a multi-film contract, Hugo Weaving claims to have little interest in reprising his role as the Red Skull, having grown tired of blockbuster work.
Mark Ruffalo is this to Edward Norton as Bruce Banner in the Avengers movie.
Don Cheadle replaced Terrence Howard as James Rhodes.
Howard Stark has been played by three different actor in his first three film appearances (Gerard Sanders in Iron Man, John Slattery in Iron Man 2, and Dominic Cooper in Captain America: The First Avenger). Somewhat justified on that last one, as Howard Stark was supposed to be about 20 years younger than he was in the Iron Man movies. Dominic Cooper returned to the role on Agent Carter, set in the immediate aftermath of First Avenger, while John Slattery returned for the opening scene of Ant-Man, which was set in the late '80s.
Zachary Levi replaced Joshua Dallas as Fandral thanks to his commitments to Once Upon a Time which is a reversal of The Other Marty, as Zachary Levi was originally cast as Fandral in Thor but couldn't because of Chuck related scheduling conflicts.
Josh Brolin portrays Thanos from Guardians of the Galaxy onwards, after Damon Poitier made a brief cameo at the end of The Avengers.
Edwin Jarvis (Howard Stark's butler in Agent Carter) and JARVIS (Tony Stark's AI in the Iron Man movies) are technically variations on the same person, in that the latter's voice and personality are based on the former. However, they're played by different actors (James D'Arcy in the TV show, Paul Bettany in the films).
James D'Arcy normally plays slimy villains, and he plays the bumbling and pompous Edwin Jarvis. He was reluctant to take the role for this reason.
Playing Gertrude: Some actors across the franchise portray characters older than they are, notably:
John Slattery portrays the older Howard Stark from content set in the 70's and after, starting with Iron Man 2. Slattery was about 47 when he began playing Howard, who was officially 53 when his son Tony was born in 1970.
Hayley Atwell portrays Peggy Carter in every time period the character appears starting with the early-40s and ending (so far) with 2014, averting Time-Shifted Actor; Peggy is born in 1919, and Atwell herself was in her late-20s when she first assumed the role. See also Dawson Casting above.
Production Posse: Joss Whedon directed The Avengers, which had two of his regulars (Alexis Denisof and Enver Gjokaj) in bit parts, his brother and sister-in-law Jeb Whedon and Maurissa Tanchauroen are the showrunners for Agents of SHIELD, and his former Buffy the Vampire Slayer writers Drew Goddard, Steven S. De Knight, and Doug Petrie are writers/producers on Daredevil. 2015 was probably when the MCU hit peak Whedon saturation.
Anthony Mackie says he wanted to play the Falcon to show the kids in his family (as well as black kids in general) that black people can be superheroes too. In general, he's been very vocal about the need for greater diversity in superhero movies.
Michael Douglas says this was his major motivation for playing Henry Pym in Ant-Man. He even took his son to Comic-Con to promote the movie!
Trolling Creator: It's been tradition since the first Iron Man to have a post-credits scene. Leading up to The Avengers, these scenes foreshadowed events that would be touched on in other films. Starting with The Avengers and continuing on in Iron Man 3, Thor 2 and Guardians of the Galaxy, these foreshadowing scenes are played mid-credits, and audiences who are Genre Savvy enough to wait to the end of the credits are rewarded with a pointless, humorous Brick Joke for their patience. The trolling has become almost poetic in its inconsistency, as inter-spliced with the Shmuck Bait stingers above are the legitimate post-credit scenes of Winter Soldier and Ant Man, and the complete absence of anything at all at the end of Age of Ultron.
Despite huge demand, Kevin Feige (head of Marvel Studios) repeatedly said that there were no plans for either a Black Panther or Captain Marvel movie... right up until the Phase 3 announcements, where not only were both movies given release dates, but it was revealed that both had been in the planning stage for a long time and Black Panther had even already been cast and has shown up in Captain America: Civil War.
Before this became reality in February 2015, Sony was keeping their options open to cash in on a Spider-Man crossover, but their new reboot has proved enough of a success that they've decided not to cut the pie. It wasn't until the negative reception the 2nd movie received and the damaging hack of Sony did Sony change their minds about doing Spidey and MCU crossovers.
And speaking of Spidey, the Oscorp building was originally going to be in The Avengers, but the film was too far in post production to include into the movie.
A Runaways movie got as far as casting calls before they realized that it's just too complicated a mythology to dump into the franchise all at once (in the comics the plot ropes in time-travellers, mad scientists, aliens, crimelords, witches, mutants and pan-dimensional god-monsters, and that's just the origin), particularly before they'd even got the Avengers fully established. Of course now that they're brave enough to make something as obscure and bonkers as Guardians of the Galaxy it's safe to put it back on the "coming soon, maybe" pile.
The Wasp was initially going to be part of the team in The Avengers, and was even included in earlier drafts of the script. Joss Whedon is on record saying that he really enjoyed writing her, but ultimately had to cut her out of the script due to there being too many characters. Then she was planned to appear in Civil War alongside Scott Lang, but it was ultimately decided to have her first appearance post-Ant-Man be in Ant-Man and the Wasp so that she wouldn't be jockeying for screen time with a bunch of other superheroes.
And now, Edgar Wright's Ant-Man, which could well have been one of the most individualistic visions within the MCU.
Early drafts of Guardians of the Galaxy had Nova as a central character. He was cut because they felt his character arc was too similar to that of Star-Lord, and because James Gunn does not like the character.
Regarding the sequel, Gunn's original script was to introduce two new characters as future team members. He decided however that one of them would need to be cut, stating that despite their being a "fan favourite", he had enough returning characters (and their own storylines) to juggle and feeling that the plot would become over-complicated if he kept them. That said, he hasn't ruled out bringing them into a future instalment, should he return to direct yet another MCU film.
Inhumans was originally going to be released before Infinity War Part II, but the unexpected addition of Spider-Man pushed it back to a few months afterward before further shakeups in the schedule (mainly the announcement of a fifth Indiana Jones film) pushed Inhumans out of Phase Three altogether with no word on a new release date.
Similar deal with about half of Phase Three. If you believe Kevin Feige, this wasn't entirely unplanned - in his words, they were considering the possibility of Spider-Man joining the MCU since October 2014, when the Phase Three slate of films was first announced. The initial announcement was "Plan A", which would involve no SM, because though they weren't sure how the chips would fall; "Plan B" was to push all the films after Guardians 2 to later release dates (save the Infinity War duology) if their bid to get the Spider-Man rights succeeded, but they didn't want to announce it in case the Sony negotiations completely broke down.
Things were adjusted again when Ant-Man proved successful enough for a sequel - it got the July 2018 release date of Black Panther (which was moved to February 2018), and Captain Marvel got a second push-back to March 2019 (presumably to prevent four movies needing a release in 2018).
Before getting the part of Falcon, Anthony Mackie said that he wanted to play Black Panther; nonetheless, he has shown zero regret about getting the part and is supportive of Chadwick Boseman being cast in the role.
Djimon Honsou was another actor who hoped to get the part, and even still held out for the possibility after his role as Korath in Guardians.
The death of Garry Shandling cut off any possibility for a future role for Senator Stern, though at least his arc has a decent closure with being arrested as a HYDRA agent.
Marvel's Most Wanted was a pilot ordered by ABC in mid-2015 to be centered around Bobbi Morse and Lance Hunter from Agents of S.H.I.E.L.D.. The season finale of Agents of S.H.I.E.L.D. provided a possible out for the characters but the characters appeared in the next season regardless. A script was written for Most Wanted and the pilot filmed in early 2016. In May 2016, ABC passed on the series (and canceled Agent Carter)... several weeks after Agents of S.H.I.E.L.D.wrote the characters off the show.
Working Title: Videos and pictures taken behind the scenes tend to reveal some humorous code names during the filming processnote Iron Man 2 used Rasputin and Murphy's Law, Thor used Manhattan, The First Avenger used Frostbite, Avengers used Group Hug, Iron Man 3 used Caged Heat, The Dark World used Thursday Mourning, The Winter Soldier used Freezer Burn, Guardians of the Galaxy used Full Tilt, Age of Ultron used Afterparty, Ant-Man used Bigfoot, Civil War used Sputnik, Doctor Strange used Eye See You, Homecoming used Summer of George, Ragnarok used Creature Report, Black Panther used Motherland, Agents of S.H.I.E.L.D. used Ragtag (season 1), Daredevil used Bluff (season 1) and Ringside (season 2), Jessica Jones used Violet, Luke Cage used Tiara, Iron Fist used Kick, Punisher used Crime, and Defenders used Group Therapy.
Wrestler in All of Us: Someone in the stunt teams or the writers' room must be a wrestling fan, as there has been a moment across the three parts of the MCU (movie, TV, and Netflix side) where an antagonist has countered a protagonist or supporting character's offense with a powerbomb:
Agents Of SHIELD: Hyde to Coulson in Season 2 (countering a triangle choke from the ground)
Jessica Jones: Will Simpson to Trish in Season 1 (the triangle choke from the ground again)
Daredevil: Jacques to Electra in Season 2 (in a slight twist, it goes from a hurricanrana by backsliding off a plane wing, transitioning into an armbar when he doesn't go down, then to the powerbomb onto the wing)
Civil War: Bucky to Natasha (a hurricanrana that simply becomes Natasha elbowing Bucky's head when he doesn't get dragged down)