Humanity

Steve

He still remembered the transformation very clearly – the Vita Rays, the needles, all the agents clustered in that bunker below Brooklyn. He knew he stopped being a normal human the instant he emerged from the shell. He did not realize he had stopped being human entirely until he woke up from a cryogenic time capsule that would have killed anything remotely human. He should not have survived and that thought terrifies him because he considers the character of the people who could have done the same and he hates those he is in league with.

Thor

He did not envy humanity and their frailty, or at least he did not used to. He thrived in his immortality for millennia and pitied those with such short, insignificant lives. He was a god, after all, and gods were quite content as they were. But then he met humanity on a personal level. As he sees these people go about their lives with such carefreeness and happiness, he wonders how long Asgardians have been lying to themselves. They were not the superior race and they had lost something of themselves when they decided they were.

Clint

He should have been the most human from the lot of them. And as far as Fury was concerned, he was. He would never disclose it. He had been highly skilled enough as it was but then he was more. It was before all of this, before Tasha and SHIELD; he had been aiming to take out a man's rival for an extremely generous reward when his existence became something that was not human. Anyone else probably would have been thankful to escape the surgery with all their functions in tact but he knew he had been changed. His aim became too precise and he could make shots by sensing body heat without thermal imaging. He didn't tire quickly and any acrobatics were increased tenfold. He only enlightened two individuals with these revelations. People as a whole were not excessively fond of discovering a masquerade as severe as his. His humanity in the broadest sense ended there; he did not see this as some twisted gift, all he saw was loss.

Tony

He thought he could continue after Afghanistan. He didn't want to pick up where he left off, keep being the Merchant of Death and exchanging money for lives, but he wanted to go back to the life he had lead. He may have deluded himself into believing it, but it had never held true.

He even removed the shrapnel and arc reactor years later but he shoves the reactor back in when he still doesn't feel human. His humanity had faded the second he had slipped on that first suit and he had no hope of truly regaining it. He felt the armor move beneath his skin, the revised Extremis floating in his blood stream, holding nanotechnology that would encase him in metal at a second's notice. No, humanity would never again be for him.

Bruce

He may never have been human in the first place. He used to think his oddity started with the gamma, but a human would not have survived the gamma long enough to acquire a Hulk. No, the logical part of his brain knows he was born weird, born abnormal. He used to hate himself for it, for being so different, but he could cultivate it. He could save lives because he was immune to the diseases that affected humanity. He would use his lack of mortality to better something because otherwise he would become the monster.

Natasha

She can barely remember real humanity as it was taken from her at such a young age. She fought and clawed her way through the Red Room (a place which could have ended her humanity by itself) to become their shining star, their prized trophy. She suffered through a crudely modified serum that killed what little of a person she had left. She flipped and spun and destroyed at rates that surpassed what any human could possibly attain. Humanity was seen as a weakness, but that had never been the case.

"We have left humanity behind. We are beyond it."