Just can't get the Avengers out of my head...
Disclaimer: None of this is mine. It all belongs to Stan Lee, Marvel, etc, and I'm making no money. First two lines are direct quotes from the movie.
"Big man in a metal suit. Take away the suit, what are you?"
"A genius, billionaire, playboy philanthropist."
It's a true answer, as far as it goes.
Tony Stark is undeniably a genius, way ahead of the bell curve. He graduated MIT with his first degree at a younger age than most kids enter it. He tinkers and perfects and creates at a level that leaves most scientists in awe. He actually synthesized a new element with a jury-rigged particle accelerator in a basement lab. Genius is an understatement.
It is that genius that permitted him to put together a compact energy source and a mechanical suit of armor out of salvaged parts while being held hostage in a cave in the desert. Maybe it was even that genius that drove him to try it, knowing there had to be a way out, a way to outsmart his captors.
He may have inherited his father's company, but he made it grow. His designs sold for millions of dollars, his charisma brought in shareholders and investors. He earned the title of billionaire, even if it wasn't entirely self-made.
Those billions allow him to upgrade his suit whenever he likes, to purchase the materials and install prototype features that would cost upwards of half a million dollars without fretting.
Playboy, well, that's hardly a secret. Tabloids and gossip columns love to talk about his latest scandal, and he rarely bothers to even learn the name of the women he has Jarvis kick out of bed in the morning. He can't remember the last time he had an actual relationship. When he was seventeen, maybe.
The only times he doesn't flirt with anything vaguely female are when he's in his workshop, or in his suit.
Philanthropist might be the title of which he is most proud. He didn't always used to be one: quite the opposite. 'Merchant of Death,' and all of that nonsense. He didn't know where his weapons were ending up and if he had known, he might not have cared, before one blew up in his face. But after his ordeal, he made a very unpopular decision, for the betterment of the world, and he stuck to it. Even in the face of public opinion and private betrayal, he kept his word. No more manufacturing weapons that might end up being used against the US or its allies.
Saving the world from villains in his suit is a little flashier, a little more active. It makes his atonement easier to measure. But the suit doesn't make him a philanthropist.
So it's a true answer, but it's not the real answer.
The answer the great Captain America is looking for is 'nothing,' 'weak,' 'nobody.' He is making the same mistake SHIELD made when they said "Ironman yes, Tony Stark no." He is differentiating between the hero in the metal suit and the mind that made it.
Without the mind, there is no suit. Without the suit, there is still the mind, hands, the will to make another. Try to take away all Tony's suits, lock them up, and given a day or two he will have them replaced, probably with an upgraded model.
Take Tony out of the suit, and you just have a lump of expensive metal. It can't do anything without him. No one else can use the suit like he can, with the understanding of its capabilities and trajectories and, yes, weaknesses. And that's not even considering the need to find a power-source, because every suit-but one, a prototype and a gift for a friend, which was hijacked and used against him—that Tony makes runs off the arc-reactor in his chest. That is a power source no one else has, because it is the product of his desperate bid to survive, and he made a virtue out of necessity.
It's not the suit that lets Tony do amazing things, it's his arc reactor and the brain that conceived it.
The suit is just armor, just a bonus, just a symbol. It's like Captain America's shield, really. It can be used to defend and attack and it's iconic. It lets Tony take hits his body couldn't stand, it lets him carry and fire more ammunition than a tank, it lifts spirits and brings hope. The latter he can do without the suit, the former he cannot.
Without the suit, Tony Stark would still be a hero.
But without the suit, Tony Stark is human.