1. They were all being effed with by Loki's spear. It was plainly stated that Loki was actively trying to get the heroes to focus on themselves and him personally. Multiple times. And temporarily, he succeeded. When his spear was removed and the shock of a friendly's death hit them they almost *immediately* got their **** together.
What You just wrote is true, but still it's explicitly shown that Coulson's death, was meant as an explanation for the name "Avengers". Hence the whole speech about avenging earth by Stark, and Fury's talk about the right "push".
Hell, even Coulson himself said right before his last breath, that "'This was never gonna work if they didn't ...". Which states that even he didn't believe that this group had the right motivation to work together to save the world. Which is just stupid. The whole theme of Avengers being a violate mix of individuals, who could never cooperate, is not just the effect of Loki's spear, but one of the major themes of the whole movie. Characters like Coulson himself, Fury and others explicitly acknowlegde the point, that their cooperation is unlikely, even in the face of an alien invasion.
Let's face it, the name "Avengers" was given to this comic, just becouse Lee and Kirby thought it sounded cool. Thats it. And Wheadon is too smart a filmmaker, to just simply leave the question of the group's name, with the answer "just becouse". So he put in that whole element of Coulson's death as a catalyst. And I am happy that he did. Like I said in the previous post, I don't hold this against the movie. But if we are to look at this theme and be honest, then it's obvious ts a bit of a forced plot design.
2. They didn't really take Loki *that* seriously anyway. He was a trickster, manipulator, and liar. Did he really have an alien army at his disposal? Apparently Stark didn't fully believe it, hence his, "right, army" comment when he saw them actually appear. "You seeing this Stark?" "I'm seeing, still working on believing..."
Are You kidding? Of course they took him seriously. You only know that Loki was a liar and manipulator, becouse You are a viewer with external knowledge of the character. To everybody else IN the movie, Loki was a hostile alien, who explicity stated his plans for mass invasion (ant - boot), and came out of a portal, that ushered into the world a giant metal machine-man that leveled a small town, the last time it was opened (something that Fury references in the movie). And if that wasn't enough, he also stole, hid away and had plans for using the tesseract, which Fury knew was a potentionally world-destroying weapon.
"
How desperate am I? You threaten my world with war. You steal a force you can't hope to control. You talk about peace and you kill 'cause it's fun. You have made me VERY desperate."
Yeah, they didn't take him seriously at all...
Also You missed totally with the Stark's quotes. Those were nothing but humorous, situational quips. Just Stark being Stark. He knew full well, what was he dealing with, and what the stakes were.