MORE SPOILERS
I do think that being able to root for the protagonists is tied very deeply to whether anyone, not just myself, can enjoy a story. The main character is someone who supposedly wants something; there is a goal, a purpose they are trying to achieve, whether they know it or not. Sometimes it's a physical achievement, but it could be emotional or metaphorical as well, but nearly every story contains something like this to drive its plot. But why would the audience even care to sit through it if they didn't want the protagonist to succeed? As soon as Stark decided to covertly do this thing that EVERYONE knew was a bad idea, he lost me, and I just didn't think the bromance between Banner and him has been built well enough to make me think Banner would go along with this clearly bad move. They knew this thing was dangerous based on the last movie, but the casual, flip way Stark decides to go rogue bugged the hell out of me. I get his character is like that, but why would everyone else trust him with the artifact then? More stupidity. Or inconsistency.
Now, I'm not opposed to mistakes being made and the characters still being relatable, but usually if they kickstart a plot they are done unwittingly. Stark knowingly creates a machine designed to protect the earth. What would logically do that? Destroying people, of course. Wouldn't have to be a genius to draw that conclusion, but the script decides to make the two scientists' brains take a nap so that we can have a bad guy.
Now, after all of this there is still a chance to explore something interesting about these people, namely remorse. But instead of focusing on Stark and Banner's incredibly irresponsible behavior, how they feel about letting the team down, and their fault in the destruction and lives lost, the story decides to spend time with Hawkeye on his family's(!) farm. Utterly came to a halt for me there, and a complete waste of time, as Hawkeye really played no major part in the macro events of this story. By the time Vision showed up I was just waiting for the end. It wasn't torturous, a couple of elements worked for me, but if this was a game of chess, it feels to me like it was being played by an amateur who made 20 moves when he could've won in 3.
Look, bottom line is that this is just my opinion, which of course every movie review is. I think there are many, many legitimate gripes with both the story and filmmaking in this movie. It just seems like it both tried to bite off too much and not enough at the same time. But I have talked to others who liked it a great deal more than myself, and I always appreciate the discussion. Some beers would've made this perfect.