It's lazy on Whedon's part.
He could show us Cap doing things that prove why he THE person who should be in charge, BEFORE the team accept him as a leader.
But he leaves it to viewers to assume that people who meet Steve can simply see it.
Jumping in on this, I have to disagree with you here. I know everyone says Banner/Hulk stole the show, but I thought Cap was the best by far. I thought Whedon wrote him pitch perfect. My reasoning:
1) In his very first scene of the movie, Fury comes to get him, says someone stole the cube, and what does he say when Fury asks him about it?
You should have left it in the ocean. That right there sets up Cap, his viewpoint and how the viewer should be contrasting him with the other characters in the movie. Cap believes they should have just left the cube alone. This comes into play later in the movie.
2) My FAVORITE exchange in the entire movie, when Cap first meets Banner:
Cap: Word is you can find the cube.
Banner: Is that the only word on me?
Cap: Only word I care about.
Again, allowing the viewer to contrast Cap's personality with other characters in the movie. Cap doesn't care that Banner has this issue. It does not define who Banner is. Loki, on the other hand, is constantly making jabs at how he is a beast.
Makes play he's still a man. It is also a not-so-subtle parallel to an AIDS victim, which is awesome. Again, this is done to show Cap's personality, let Banner see what type of person Cap is, and allow the viewer to compare him with others in the movie, specifically Loki.
3) Stark comes in because he wants the cube. He and Thor are used to doing whatever they want, they have never really had to answer to anyone, so they start fighting because they each want to do their own thing. Who is the one with the level head? Cap. Cap comes and tells them to knock it off, even lectures Thor saying stop messing around. He even blocks Mjolnir, which is surprising to Thor. Maybe Thor should take a step back and listen to this guy.
4) When they get back on the ship, Cap is the only one who is still trying to figure out why Loki let himself be captured. Banner says don't worry about Loki. Stark is trying to turn Banner into the Hulk. But Cap is the one who says I want to know why Loki let us take him. Cap is the one who says knock it off to Stark, and then he hones in on Loki's plan.
I think Loki's trying to wind us up. Another example there to show Cap's personality, in this case, contrast it with Tony, who wants the cube for himself, and Banner, who says not to worry about Loki.
4) After everything happens on the helicarrier, after Coulson dies and Loki escapes, Tony realizes he fell for Loki's trap, as Cap put it, trying to wind them all up. He also realizes that Cap was right the whole time, which significantly changes how Tony views Cap. When they first meet, Tony isn't impressed by him, going so far as to insult him, saying everything special about him came out of a bottle. Well, no it didn't. Tony fell for Loki's trap, Cap didn't. Cap was on him from the get go. This is apparent in the next scene when Tony confronts Loki:
...a supersoldier, living legend who kind of lives up to the legend...
This is the big change in Tony's character. Half way through the movie, Tony was insulting him, saying they should have left him frozen. Now though he has seen what makes Cap special. Cap has proved to Tony why people revere him so much.
In addition, Whedon makes a note to put Cap's courage on display several times, from the him being the first one to take on Loki,
alone, to when he was risking his life by jumping across gaps all over the helicarriers engine to fix it. In the final battle, when he is running through fire and explosions to go save a bunch of people. Finally the part where he actually gives orders to the NYPD, which is a nod to the confusion/miscommunication which took place among first-responders on 9/11 (AWESOME).
And then at the very end of the movie, when the council asks Fury where the cube is:
...it's where it belongs, out of our reach. So Cap was right the whole time. The cube should have been left in the ocean; if it had, all of this would be avoided.
Cap was easily my favorite character in the Avengers, Whedon went to great lengths to display his character, in particular that the insult Tony uses, how everything special came out of a bottle, is wrong. His courage, his wisdom, and his moral compass did not come out of the bottle, and those are his true superpowers. All of this exemplifies that while he is not he most powerful or the smartest, he is the wisest, which is why he leads the Avengers.
So calling it lazy on Whedon's part is dead wrong. If you think it was lazy it was because this stuff was going over your head. There is no way I could have pulled all of that from the movie with Whedon writing him in a lazy fashion.