7. Bucky
The way the Russos handled The Winter Soldier in their Captain America sequel was great, but Marvel dropped the ball with Bucky in a major way in The First Avenger. In fairness to the the studio, there was a lot of story they needed to tell in this final movie before the release of The Avengers, but to make Bucky such a forgettable character that a lot of casual viewers had forgotten who he was by the time he returned is shameful. While The First Avenger certainly got some things right (Bucky protecting his scrawny best friend from a bully for example), he still felt like an afterthought, and all of his scenes combined failed to make a lasting impact because they simply didn't focus anywhere near enough on their friendship; we were told they were friends, but barely saw it. What the movie needed was a less time with the irritating Howling Commandos and more spent focusing on Captain America and Bucky teaming up, something an unsatisfying montage tried and failed to show. However, it's Bucky death which is by far Marvel's biggest screw up. It happened too quickly and without a significant amount of drama, and it's baffling that they would choose that moment rather than having him go down battling the Red Skull in that final battle. Bucky sacrificing himself to stop one of those bombs would have been a nice nod to the way his comic book counterpart was taken out and would have made that fight between Steve and villain infinitely more exciting. It's great that they've got him right now, but his origin story was botched.