4. Lack of Spider-Man
Well, this one's just a pipe dream and there's no getting around that, but wouldn't it be amazing to see Spider-Man show up as part of The Avengers? Depressingly, the Oscorp building was originally planned to show up in the New York skyline in The Avengers, but they couldn't get the render finished in time, so it was left out. It's a shame, as this at least suggests that Sony and Marvel Studios
are on the same page in some capacity, despite protestations that they are not. The problem is that cutting through all of the bureaucratic nonsense and having the two studios agree on terms is just a legal minefield; neither studio wants to be seen as conceding to the other, and as a result we'll almost definitely never see Spidey join Iron Man and friends.