The Marvel Cinematic Universe has grown to become the biggest movie franchise in the history of the industry, but it certainly had a head start thanks to the decades of comic books that preceded it. Building from characters and stories that already had huge fanbases, Marvel Studios had a lot of source material to draw from.
However, the franchise has always taken certain creative liberties with these properties, often either adding new details or omitting important aspects from the original comics. This can either be a blessing or a curse, depending on how the audience perceives such a change.
Iron Man 3's Mandarin for example, was one of the most divisive aspects of the entire MCU so far, as it offered a radical departure from the source material, but there have been more than a few times such changes have been positively received almost unanimously by audiences.
From iconic story lines, to character histories and backstories, and even their design and costumes, there is seemingly little that Marvel Studios are not open to changing. At this point, it's pretty clear that the franchise knows what its doing, and most decisions to ignore the source material are made for very good reasons.
10. Changing Thanos' Motivation - Infinity War/Endgame
There can be no doubt that Thanos is the greatest villain that Marvel have ever brought to the screen. He was powerful, intelligent, and most importantly, had a strong motivation for doing what he was doing.
This was done expertly by all involved, as it could have gone in completely the other direction had the writers decided to keep closer to the source material.
Throughout his history in the comics, Thanos has always had close ties to the infinity stones. He searched the universe to find them and wanted to assemble them all in his gauntlet to snap half of the universe's population away, but it was his motivation that was different. While the MCU version was doing such an act to save the lives of those he didn't dust, the comic book version of Thanos simply wanted to impress Lady Death.
The personification of death became something of an obsession for the Mad Titan, and he wanted nothing more than her attention and affection. He sought to serve her by sentencing half of all life to death, a story arc that would have been far less compelling when translated on screen.