The story behind a movie is what drives its success. But writing a great story for one movie is already a challenge. Writing a great story for several movies in a series is even more challenging. It seems that Hollywood already has trouble coming up with one creative idea, let alone several. Meanwhile, Marvel has written twenty-two movies, featuring hundreds of different characters, each with their own storyline that all come together under one overarching tale. The level of planning and creativity needed to construct this kind of epic is unthinkable. Marvel movies show incredible internal consistency in its characters and worlds, while at the same time charting how those characters and worlds grow and change. It is a vast project of incredible quality. Fans have long been captivated by the journeys of beloved characters over the course of several movies. They have watched Tony Stark grow from an irresponsible playboy to a man conscious of his duty to protect the world. They have watched Captain America grow from just another scrawny Brooklyn kid to a symbol of heroism admired the world over. The stories that Marvel tells have so much thought and emotion put into them, and they captivate audiences of all ages. For this reason, they deserve to be considered as great cinema.
Too much credit is given to how "deep" Marvel plots are. In fact, they're all very simple. There's a bad guy trying to take over the world (or universe), and there's a hero who needs to learn how to beat them - that's it. It might be a nice watch the first time, and there may be minor differences here and there. But overall the basic structure remains the same, and the story each movie tells is far simpler than fans may want to give it credit for.