I watched it too, and I will reveal a spoiler for you all.
The dog dies.
The alien race is NOT the Skrulls, but the "Chitauri"
Pepper Potts (from Iron Man) makes a cameo
As does Stan Lee, but I forget what it was (it's towards the end, I think he's one of the army generals)
Bruce Banner reveals to the team that prior to these events, he tried committing suicide by shooting himself in the mouth, but the "other guy" spit the bullet out
Loki is 10x more brutal & ruthless than he was before
Hulk & Thor do battle, and during the skirmish, Hulk attempts to lift Mjölner (Thor's hammer), but fails
Hulk & Thor are about equal in strength
Hawkeye & Dr. Selvig (Thor) are under Loki's mind control for about half of the film
War Machine (Iron Man 2) does not make an appearance
The events of "The Incredible Hulk" are referenced, as Bruce proclaims that the last time he transformed (a year prior), he leveled Harlem, NY
During the attack, the teammates ponder how Bruce will be able to control Hulk, which Bruce replies, "I'm always angry"
The Tesseract is never referred to as the "Cosmic Cube," as in the comics
The Tesseract is used via Gamma Radiation & Dr. Selvig's help to create a portal, which brings the Chitauri to Earth & begin the invasion
Before all this, Loki is seen making a bargain with a mysterious alien. The bargain involves the alien giving Loki dominion over the Chitauri to invade Earth
Captain America never yells "Avengers Assemble!"
Towards the end, Hulk confronts Loki, which Loki replies "You can't kill me, I am a God." Hulk proceeds to trounce Loki, literally calling him a "puny God"
Captain America yells "Hulk, SMASH!" which he does
If you can't tell by now, Hulk steals the show
Agent Coulson confronts Loki (Loki is imprisoned in a special room on the SHIELD helicarrier that is designed to contain the Hulk) with a weapon that was back-engineered from the Destroyer (Thor). He proclaims he'd like to see what it does, but before he can fire it, Loki stabs him with his lance, killing him
Black Widow uses the weapon on Loki, blasting him off the Helicarrier. Right before Coulson dies, he proclaims "So that's what it does"
The Government, in an attempt to stop the invasion, decide to nuke Manhattan despite Nick Fury's refusal. Iron Man intercepts the missile & delivers it into the portal created by the Tesseract, effectively destroying the alien stronghold
Iron Man, having run out of power, narrowly escapes the portal. As he plummets to Earth, Hulk rescues him middair
After the credits, the mysterious alien & Loki are seen again. The alien is in fact a servant of THANOS. He tells Loki of his failure, yet he will have his second chance. THANOS then looks towards the camera and smiles