It's the failure of Steve Rogers as a character that's most responsible for sinking Captain America: The First Avenger, but the movie's a mess in other respects, too. Too many elements trying to gracefully dovetail, it comes off more like five doves smashed into each other ass first, turning their tail feathers into a bloody grey mess.
I can see how it could have been a better movie with some of the more admirable motives visible in the story. Steve Rogers being the weak and therefore wise and gracious man coming to represent the best of American ideals, uniting his comrades in the face of Nazi horror. Stanley Tucci as the scientist responsible for Rogers' transformation has some lines I really liked about how their manufactured superhero couldn't be a "bully". I liked how Rogers wanted a chance in the army not because he wanted to kill Nazis, or anyone, but out of a basic sense of decency.
Unfortunately, it's very difficult to get a handle on Rogers as a character. One of the most notorious problems with fundamentally good characters is that they can be boring. Nobody's perfect, which results in a character being portrayed as perfect coming off as nobody, basically white noise. Chris Evans isn't as annoying as he is in the Fantastic Four movies, but his lacklustre performance here doesn't help.
The special effects early in the movie for skinny, pre-serum Rogers are amazing. Evans' head is grafted almost seamlessly onto a smaller man's body. Unfortunately, Evans still talks with all the confidence of a big guy who doesn't get beat up all the time. I'm not sure it helps anyone to say persistent, extreme physical abuse ought to just roll off a guy's shoulders. I understand the idea is to celebrate human dignity surviving hard knocks but the movie doesn't seem to know what hard knocks means. Just from the fact that the Nazis are sidelined by a group of faceless Sci-Fi ninjas called Hydra says the movie's made by kids who've been left to play in the gun shed.
I guess the idea is to give the characters an adversary it's easier to believe whose bullets can always be dodged and who can be taken out with a thrown shield. But any attempt to downplay a murderous organisation trying to take over the world comes off as insulting when you hinge the whole movie on the characters' struggle with them and whether or not human decency can survive the struggle. Captain America: The First Avenger is a prime example of how trying to play it safe can end up coming across ten times as obnoxious.