Arguably, the greatest thing about the MCU was Tony Stark, aka Iron Man. Perfectly brought to life by Robert Downey Jr., in a performance that not only revitalized his career but also cemented his status as a Holywood superstar. Fans could not get enough of Tony Stark as a character, especially when he was interacting with others throughout the Marvel Cinematic Universe.
But the Tony Stark from the movies is very different from the Tony Stark of the comics. In the comics, Tony Stark is a very complicated character, as he is undeniably a hero but his personality is rough enough that saving lives isn't enough to get people to like him. He's also verged the line between villain and hero, as often seen in stories like Civil War and the Extremis Virus. Film goers prefer their characters to be a bit more likable than their comic-loving counterparts so it only makes sense that some things had to be changed. But not all of these changes hit the mark.
10 Good: His Relationship With Pepper Was Expanded
In the comics, Pepper Pots and Tony dated/had a brief relationship with each other. But Pepper originally had a relationship with Starks bodyguard and driver, Happy Hogan. Pots and Hogan got married and even took a break from working for Tony.
Whereas in the films Pepper dates and even marries Tony. Which is one of the reasons why he chooses to improve on himself as a person.
9 Bad: Removing The Arc Reactor
This was a focus point and important factor in Tony Stark's life throughout the history of the comics. It was also a main plot point in the films. Both mediums parallel each other throughout the first Iron Man film. But by Iron Man 3, Tony removes the metal fragments in his heart, removing the purpose of the arc-reactor. This allows Tony Stark not to be defined by Iron Man.
But in the comics, Tony Stark and iron Man basically merge, and the arc-reactor becomes an essential part of who Tony Stark is; which affected the stories written about him for decades.
8 Good: Dealing With His Alcoholism
In both the Prime Universe and Ultimate Universe, a big aspect of Tony Stark's character is his alcoholism. He uses it to deal with the various skeletons in his closet, from having to live with the arc-reactor, to his relationship with his father, as well as knowing his weapons killed so many people.
In the films, they show his alcoholism in Iron Man 2, but he is able to overcome it. He chooses to become better rather than continue drinking his life away and being Iron Man; which makes him a better Tony Stark than the comics.
7 Bad: Making Jarvis An A.I.
A big part of the Avengers comics was Tony Starks butler, Edwin Jarvis. He was an honorary Avenger, since he assisted the heroes with their lives and provided things like advice and emotional solace when they were down. He was also crucial character in several sotrylines, like the Masters of Evil, the Secret Invasion, and the Axis stories.
In the films, Jarvis was turned into an AI created by Tony Stark and voiced by Paul Bettany. He just works as computer assistant to Tony Starks genius.
6 Good: Mentoring Peter Parker
When Spider-Man was introduced in Captain America: Civil War, the writers of the film decided to set-up Tony Stark as a mentor to the young hero, not only giving him advice but also pushing him to be more than just the technology he was wearing.
This not only pushes Stark to care about someone other than himself and Pepper, but it plants the seeds of father-hood he would need for when Morgan Stark comes into his life. Plus, it creates the father figure MCU Peter Parker was missing.
5 Bad: Not Making Him Director Of Shield
A big plot point that led to a long list of great stories was Tony Stark becoming director of SHIELD. It was a perfect match, since Stark was a weapons manufacturer and hero, coming off the rails of the Civil War event. This was primarily done because of Starks involvement with the Super-Hero Registration Act, which led to the Civil War.
In the films, Tony isn't as involved as the comics, and SHIELD remained under differnet management. Had Tony become director of SHIELD, who knows what great storyline fans could have gotten.
4 Good: Toning Down His Villain Side
A piece of lore that was easily noticeable during the Civil War event in the comics, is that Stark bounces between being a villain and a hero on occasion. From having no emotional consequences to arresting or even fighting his friends during the superhero Civil War, to ejecting Hulk into space against his own will, comics Tony is a bit evil.
In the films, Tony Stark was written to be a much more compassionate and understanding man. He is bothered when other people are put in danger, especially those close to him, and tries to find the best solution for everyone. Though he can still be stubborn and rough around the edges, he is much softer than he was in the comics.
3 Bad: Removing Arno Stark
The creation of of the character of Arno Stark was a bit controversial, solely because it retconned Tony Starks lineage as being Howard Starks real son. But Arno Stark helped humanize Tony Stark as a character, especially since he was such a jerk throughout most of the comics.
Had this plot point been brought to the MCU, fans could have had a chance to see Tony Starks humanity show in a way most fans hadn't seen before. This is someone who is related to Tony Stark and is still alive and doesn't hate him. It would have been a great addito to Starks life in the MCU.
2 Good: Creating Ultron
In the comics, Ultron was created by Hank Pym, aka Ant-Man/Giant-Man, and was a product of Hank's inner hatred for humanity and it's inability to be better. This made him an absolutely evil-machine that only wanted to remove humanity from existence. In the films, Ultron is a construct created with he assistance of the Mind-Gem and is a reflection of Tony Starks need to protect and control everything.
By making him a creation of Tony Stark, we have an ultorn that is much more human, because his reasons for killing humanity are out of the desire to protect it, rather than hatred of it. Plus, it led to the creation of Vision.
1 Bad: Killing Him Off
In the comics, Tony Stark is still very much alive and kicking. In the films, as anyone who has watch Avengers: Endgame knows, Tony stark is no longer with us. Although this was expected, since the actor playing the character would never be replaced and film-contracts do expire, this is still huge loss for the MCU.
Many fans watched the films solely because of Tony Stark, a character that essentially started the MCU. So to see him leave, even if in a way that does the hero justice, is a huge loss.