You can think this stance is Steve being stern, or showing off his arms (which I’m sure the producers of these movies love), but it’s really just Steve protecting himself.
Because what does Steve know how to do? Lead. Fight. He’s a soldier.
What is he doing in all these GIFs? Talking. Listening.
When he talks to Peggy Carter in Captain America: The First Avenger on their way to the lab where he’ll ultimately be transformed into the guy whose arms you can’t stop staring at, he remarks that their conversation is the longest he’s ever had with a woman.
You think Steve’s conversations with men were any longer? Outside of Bucky, MAYBE?
From personal experience I can tell you that there are different ways to cross your arms. What we see in this GIFset may just be a personal tic for Chris Evans that was accepted as a character choice for Steve Rogers. As someone who struggles with anxiety myself, crossing my arms is a way of protecting myself while trying to not look like I’m protecting myself. So maybe that’s what Chris is doing here.
Either way, look: he’s not pointing his elbows. He isn’t making a show of prowess with his arms; he’s guarding himself. This posture makes his arms wrap around his stomach and support his spine so that it’s not as difficult to stand-up straight–which is something that, when you do it, forces you to expose yourself, physically and emotionally.
This can make you look “tough” and “together” when really you’re just holding your body together.
This is to say nothing of the fact that each and every one of these moments in the movies are moments of vulnerability, especially the reveal of Jarvis’ destruction by Ultron in Avengers: Age of Ultron (HELLO, Jarvis is artificial intelligence, a computer program, helpful and witty though he is, that Steve has sympathy for), comforting Wanda after the events in Nigeria in Captain America: Civil War, and, later in the same movie, talking to Bucky for the first time since they fought on the crashing Helicarrier in Captain America: The Winter Soldier.
Steve has broad shoulders and big arms; he is a specimen (to quote Erin [Aaron?]) of physical prowess; but let’s not forget that Steve’s heart was bigger than his body before his transformation, and was bigger after as well.
Steve never cared much for his body, which is why he had no problem standing up to that bully in the alley behind the movie theater, or going through basic training in the military, or throwing himself on what he thought was a live grenade to protect others. His heart is so big that he acts without thinking, throwing his body into danger to save others. But when no one immediately needs saving, when he just has to stand there and… be Steve, when his heart is just… exposed, what does he do?
Loki is the Philosopher King 