Another Dimension: Or at least a "new universe", if Erik Selvig's comment in The Avengers is anything to go by.
Arm Cannon: The cannons that some of the Chitauri have (including the eponymous Item 47) seem to be fit around their arms to fire one-handed. They, however, do not seem to actually be the arm, since an arm falls out when Captain America chops it off of one of them.
Awesome Personnel Carrier: Their main transport is the Leviathan, a gigantic flying serpent-like creature that carries hundreds of soldiers and can fly through skyscrapers. It takes huge amounts of explosive or electrical damage to put one down. Or one Hulk punch and a several Stark missiles.
Bayonet Ya: Their infantry sport them on their rifles.
Blood Knight: They grow restless when denied war, to the point where they'll attack each other. The tactical problems with this are summed up in a deleted scene:
Frickin' Laser Beams: Two varieties. One is fired from a thin spear/rifle used by Chitauri foot soldiers. The other is a powerful Arm Cannon that's mounted on Chitauri commanders and units riding in hovercraft. Both fire blue colored beams.
Humanoid Aliens: Barring the freaky double-thumbs and evil-looking skull-faces.
Easily Conquered World: Believed Earth would be one of these, and that the entire Earth would surrender the minute the Chitauri landed.
Humanoid Aliens: He has the same general shape as a human (and of course is played by a human actor), but the details are very different. For starters, he has two thumbs on each hand. It's not really clear if he's of the same race as the Chitauri.
Smug Snake: He's certain that Earth will surrender the instant his Chitauri launch their assault (see his quote). He wisely becomes more cautious of the humans after the Avengers repel the invasion. This might qualify him as a subversion, as a key characteristic of a Smug Snake is an inability to learn from their mistakes, a weakness he clearly does not share.
Teeth-Clenched Teamwork: From the start, he shows annoyance over working with Loki. Though, it was ultimately Thanos to make that decision, and gave him both his army as well as the scepter.
A pioneer of the comic industry, Stan the Man is the creator of many of Marvel's most famous heroes and makes a cameo appearance in all of Marvel's movies.
As Himself: Except perhaps in Iron Man 1 and 2, where he may or may not have been Hugh Hefner and Larry King. Definitely not himself in Captain America, since he would been about 60 years younger at that point in time.
Butt Monkey: Mistakes a Senator's aide for Captain America, Tony Stark can never get his name right, gets sick from a gamma radiation laced soft drink, has the back of his truck ripped off, and misses all of the Avengers' action in New York.
Creator Cameo: With the exception of his appearance in Captain America. He did not create that character but was responsible for bringing him into the Silver Age, as well as creating Cap's now-iconic shield throwing.
One half of the Outlaw Couple that formed following the Battle of New York. After getting pitched the crazy idea to rob banks with his girlfriend, he went along with it reluctantly, but deep down is grateful for the chance to finally live out their dreams.
Anti-Villain: Just an ordinary guy reluctantly robbing banks to fix his life's problems.
Boxed Crook/Cut Lex Luthor a Check: At the end of Item 47, he's given a job at SHIELD in their Research and Development department's "Think Tank" due to his skill with Chitauri weaponry.
Claire's had a hard life of bills, rejected dreams, and alien invasions. But when a Chitauri gun literally falls out of the sky as if as a present, she sees the chance to finally take matters into their own hands and gain the means to live like they want to.
Anti-Villain: Just an average woman with bills to pay who thinks she could solve her problems with bank robbing.