When last we saw Loki, he was falling from Asgard into space. Now we see him receiving a glowing scepter, and ready to wage war upon mankind... And he's going to use the Tesseract to do it!
Nick Fury realizes it's time to bring the team together!
In bringing the team together, you get a whole lot of big personalities onto a screen that might not be big enough to hold all of them. Iron Man, Thor, and Captain America have all headlined their own movies. (So has the Hulk, but with a different actor) They're used to doing things their way and on their own. And so you get the inevitable learning to work together/heroes fighting each other scene. Which was actually about as awesome as it was predictable, and made even better with Loki just kind of watching from the sidelines with that bemused smile.
Grumbling, squabbling, and fighting continue until Loki escapes and in the process kills the beloved Agent Phil Coulson. That gets the team to work together... with a little help from Fury showing off some of his Captain America trading cards, conveniently bloodied...
Then there's the big fight as Loki's portal is opened, and the chittari army arrives in midtown Manhattan, which the Avengers manage to defeat, mostly because the shadowy world council decides to nuke NYC, and Iron Man tosses that up into the portal. It's fun, and spectacular, and interesting seeing how these wildly different heroes all use their skills and powers to face the enemy. One thing that stands out is how much focus is spent not just on defeating the bad guys, but on saving as many lives as possible. You don't see that a whole lot in other movies like that.
The Avengers does a really good job making sure that everyone gets not only their fair share of screen time, and opportunities to look cool, but more importantly, the chance to interact with everyone else, often one on one. It really gives the characters the space to show their personalities. This is doubly so as everyone interacts with Loki! In such a busy movie with so many characters it's fantastic that they took the time to do it, and that's what helps makes the MCU so unlike everything that's come before.
Next Up: Tony Stark's PTSD! (aka Iron Man 3)
Side note: The Avengers Assemble album might be one of my favorite mixes of great songs to get stuff done to. Crank it up, and you'll want to get moving!