Disfigurment
Tony loves to flaunt his arc reactor. He let the whole world know it's there, embedded in his chest. He talks about it constantly. He wears shirts that the blue light shines through, and if you're one of the lucky people he trusts, you get to see him wearing t-shirts and tanks with a hole cut out to show it off.
It's his legacy, his baby, his greatest creation, the peak of modern science unreachable for everyone else. Arc rector, it's – the future.
That – exactly like Tony's wide smiles, pats on everyone's backs and his cockiness – is nothing but a smokescreen. It takes a long time to realize that, though, and most people grow bored with Tony's antics too soon.
Steve has only understood that, really understood, when he innocently asked Tony if they could use his jacuzzi together, long before they even made love. Tony said no and refused to explain why. By being a stubborn bastard, Steve learned that he'll let you see the blue circle that'll blind you, but he hates letting you see the scars around it.
The scars are the past.
They are ugly and painful, Tony says, but Steve loves them just the same.