Spider-Man: Despite not having an origin story in the MCU, we've seen him explore his powers (and how they work) across 5 previous movies.
Hulk: Super Serum + Gamma Radiation. They explain how his "power" works. Anger = steroids, basically. Easy to comprehend.
Cap: Super Solder Serum. Explained what it does (peak human physical condition). How is that misunderstood?
Iron Man: When did he fall 20,000 feet and hit something? Never. He's flown away in the Iron Man movies, and been caught (Hulk, etc) in other movies. This is just poor misdirection.
Quill: Held the Stone for a few seconds, would've died if not for all Guardians absorbing/spreading the damage. The movie itself was also good, and they do hint at Quill being something more than Human (half Celestial). Pretty obviously decent writing .
Black Panther: Given, however, it was a good movie with good storytelling. Kinda gets a pass for being a great movie.
Captain Marvel: How does he McGuffin-fueled power work? She has power from the Space Stone? How does that grant her "photon blasts"? How does she control power within seconds of being unleashed, despite never exploring power? What are the limits? All of the previously-named heroes have defined limits? Why does anyone want an MCU "Superman"..?