Well, I probably wouldn’t read too much into it, but since you’re inviting me to overanalyse—
Natasha is someone who can’t rely on brute strength when the cards are down. She is a formidable fighter and she knows this even if those who look at her do not but that isn’t enough. She uses everything, every piece on the table, even some that wouldn’t normally in play. And to do that, to fight like that, to exist like that, she has to be aware of everything.
She knows Coulson, she knows the helicarrier, she’s spent the last scene testing Dr. Banner as he tests her. But Captain America is someone she really only knows from a file, or from stories people tell, propaganda. And now, here he is, awake and breathing. Of course she’d size him up, this new piece on the table, and wonder if the man fits the legend. Wonder what Fury’s got planned for them all, really, how they can fight, who she can trust. And in the floating military industrial complex setting, when they’ve all been brought in on a high-stakes supermission and they all know it, I don’t think Natasha feels she has to hide that. That she’s observing. That she’s sizing this all up.