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I personally tend to be a character-driven viewer, and the simple fact is that this movie didn't have a lot of character development. With such a wide cast, most of the movie was taking established characters and having them meet/prepare for a fight/fight. The vast majority for character development was on the part of Thanos himself - and while I love a believable and well-developed villain, it was also painfully generic and obvious.
Only wish there was more Steve Rogers
You didn't feel there was character development on the part of various Avengers? Although I agree not every one of the dozens of heroes was given an in depth role (because you can't), I felt most of the characters evolved over the course of the film. For characters they did choose to focus on--the Scarlet Witch and Tony Stark for example--I thought there was a great deal of change across the film.