The Marvel movie universe has brought more fans than ever into the Avengers franchise, which is actually ironic... considering the comics they're based on were created to do exactly that. A plan that worked so well, more fans may know the re-imagined version of Marvel's heroes than the original.
Back in the late 1990s, executives at Marvel Comics were concerned about the lack of young people interested in superhero comics. It was decided that the best way to get people interested in the universe would be to start a new, modernized continuity, separate from the main books. This was the start of the Ultimate Universe lives on -- and its legacy has been highly influential on the rest of Marvel -- especially the Marvel Cinematic Universe.
Much of the groundwork for the MCU was drawn not from the mainline comics but the Ultimates instead. This means that many aspects of the MCU that people might think as essential or adapted directly from to the comics actually aren't. While the runaway success of the MCU means that Quicksilver/Scarlet Witch romance plot in the movies-but aside from that there were a number of good ideas from the Ultimates that have made their way into the mainstream consciousness by way of adaptation.
Nick Fury
When Mark Millar first wrote The Ultimates in 2002, he modeled his version of Nick Fury off of Samuel L. Jackson. Jackson, a comic book fan, called Marvel and agreed to let them use his likeness -- so long as he got to portray him in any possible movie adaptation. The result? Instead of being a Caucasian, James Bond-esque superspy and WWII veteran, the Nick Fury of the movies has all the charisma and badass attitude of Jackson's classic movie roles. Seeing as how the MCU has now become one of playing Nick Fury for another decade, at least.
Iron Man
Arguably the most important change, the his character development in the films greatly. While it's true that the Stark of main continuity has struggled with alcoholism (and he even tried to help Carol Danvers quit drinking) his personality in the comics was much more stereotypically heroic and less hedonistic prior to the first Iron Man film. This is also due in part to Robert Downey Jr.'s excellent portrayal of Stark, a casting decision that's accidentally foreshadowed in The Ultimates #3 when Nick Fury jokes that he's gotten "his nose smashed more times than Robert Downey Junior."
Captain America
the scene from The Winter Soldier where The Avengers. Yet for most of his publication history Captain America's connections to the 1940s wasn't treated with the same level of importance as it is in the movies. The Ultimate Universe version of Steve however was portrayed as someone who struggles to adapt to the 21st century and who quietly laments the loss of his past life and friends. Of course, Ultimate Cap was also a chauvinist who carried with him casual 1940s style bigotry in the name of "realism" but that interpretation of the character was smartly not adapted into film.
Hawkeye
Hawkeye is extremely different in the Ultimate universe than in 616 or the films, being much more hotheaded and wearing a different costume with a mask. But the reveal of Hawkeye's family in decides to become the sword-wielding Ronin Ultimate Hawkeye decides to trade his archery equipment for a pair of guns. Hawkeye's transformation and portrayal during Endgame was likely inspired by his development in the Ultimate Universe, only the Ultimate Hawkeye became even more violent and depressed than the movie version.
The Hulk
While the movie version of Banner tried to replicate the super soldier serum. Other than that the MCU version of the Hulk didn't take much inspiration from the Ultimates. This is probably a good thing considering that Ultimate Hulk is a sexually frustrated monster (and occasionally a cannibal) rather than the more nuanced Jekyll and Hyde figure from the main universe. Just like Captain America's Jingoism, some ideas from the Ultimate Universe are best forgotten.
Spider-Man
The Ultimate Universe began with Ultimate Spider-Man and so it's fitting that the MCU take so much influence from it for their version of focus on Peter as a High School student as well.
Another moment of inspiration from the Ultimate universe is Peter's mentor relationship with Iron Man, a relationship that's crucial to MCU Peter but actually originated from Ultimate Spider-Man. Aside from power and responsibility one of the major themes for Spider-Man as a character in the main universe has been him maturing to adulthood and becoming the kind of hero that others can look up to, whereas Ultimate Spider-Man chose to emphasize Peter Parker as a young person struggling to balance his normal life with his Superhero persona. The MCU chose to go with the latter interpretation of Spider-Man, which just goes to show how much the MCU owes the Ultimate Universe and how different the movie versions of these characters ended up being from the original.
