As a core member of the Avengers with patriotism that rivals Superman, Captain America holds a key role in the Marvel Cinematic Universe which was and is responsible for a huge surge of new interest in superheroes.
With the premiere of Captain America: The First Avenger back in 2011 to the much anticipated Captain America: Civil War that will kick off the MCU's next phase of films, everyone's favorite Star Spangled Man has come a long way when you take his comic book history into account.
Captain America was created by Jack Kirby and Joe Simon in 1941 to basically serve as a propaganda tool. This was around the time that World War II was starting to gain serious traction, and although the American government wanted to remain neutral, Kirby and Simon felt differently. In fact, the first issue that Captain America ever appeared in featured him soundly punching Adolf Hitler in the face.
His adventures back then often pitted him against enemies of the same caliber, such as Nazis, the Japanese, and other members of the Axis Powers. Because of this, he was the perfect representation of what America was supposed to stand and fight for.
More than that though, Steve Rogers wasn't just some macho guy hopped up on super soldier serum; before being given the serum, he was the frail, sickly son of poor Irish immigrants. When he wasn't sketching in a notebook, he was trying to do what was right, even if it meant getting hurt.
"Captain America: The First Avenger" nails down pre-serum Steve Rogers perfectly. When he attempts to enlist in the army despite his many ailments, Dr. Erskine--the man responsible creating the super soldier serum--asks Steve if he wants to kill Nazis. Instead of enthusiastically saying yes, Steve says,
"I don't want to kill anyone. I don't like bullies; I don't care where they're from."
In this key scene, Steve is aptly pointing out that if power is in the wrong hands, it can and will be abused, thus creating bullies. It's something he clearly knows after a lifetime of being shunned and beaten down because of his small stature and weakness.
Erskine recognizes this in Steve when he later tells him,
"The serum amplifies everything that is inside, so good becomes great; bad becomes worse. This is why you were chosen. Because the strong man who has known power all his life, may lose respect for that power, but a weak man knows the value of strength, and knows...compassion."
Steve takes this to heart, going so far as to sacrifice himself so that a plane full of weapons of mass destruction won't be unleashed on the world. And even though he's a soldier, he doesn't blindly follow orders if it means taking away the basic rights of people.
In "Captain America: The Winter Soldier" when S.H.I.E.L.D. director Nick Fury tells Steve about Project Insight--a plan comprised of three heavily armed heli-carriers designed to preemptively neutralize threats--Steve openly disagrees with the project, reasoning that keeping the world safe shouldn't mean arresting it in fear.
Again, he addresses the issue of power imbalance. Even if it's meant for the good of the country--or in this case, the world--Steve strongly believes that taking away someone's freedom is not an option.
Freedom is something that Captain America fights for over and over, whether it's for his friends, his country, or his earth. As a call to arms in "Captain America: The Winter Soldier," he even says:
"The price of freedom is high. It always has been. And it's a price I'm willing to pay. And if I'm the only one, then so be it. But I'm willing to bet I'm not."
In the end, Captain America doesn't actually represent America. He represents the moral ideal of what America should be.