Ok. So this all seems kinda far-fetched to me now, but it was extraordinary to watch it play out when I fully believed that these were actual themes of the movie. I would say that the main topic of the movie was the nature of the ego.
Puzzle pieces:
1. One of the first lines in the movie that caught my ear was Fury's response to his side kick asking why bother saving the presumably doomed earth. Fury's response "Until such time as the world ends, we will act as though it intends to spin on", which I interpreted to be representative of spiritualism/brain-in-a-vat, e.g. "Until such time as hyperspace or an a hyper-awakened state is proven to exist, I will go on with my life as though it does not". In other words "It is reasonable for me to act on the assumption of the most positive interpretation of my life heretofore".
2. Another quote from the movie that I found to be very powerful was spouted by Loki early in the movie: "Freedom is life's great lie, and once you accept that, you will know peace". This of course brought on all sorts of ideas about free will. This idea in combination with the power of the tesseract (#3 below) was a statement that the ability to discern truth is dependent on factors that we cannot control and thus our ability to find real knowledge and truth is an illusion, and our ability to control our own minds is out of our control.
3. Loki has some sort of remote viewing experience where he is discussing the terms of some agreement with some evil alien that exists in a far-away realm. The being threatens Loki and Loki reminds the alien that he does not exist unless Loki brings him back to our world. To me, this symbolized the fact that thoughts within our mind (indeed sometimes dark and evil thoughts) do not affect how we are perceived by others until we let them out by telling others or by acting based on them. In other words, my true personality is a secret that only I know.
4. The tesseract shows people some amazingly wonderful "truth" that hypnotizes them, thus taking control of their minds. This symbolized the tendency of people to latch on to ideas as truth, and how this tendency ultimately clouds our minds. My further interpretation at the time: "The main heroes don't need this truth, of course, because their sense of truth comes from within".
5. Though there is presumably much of it going on in the movie, there are few actual shots of everyday people in the city streets scrambling for safety. Every time I saw a scene like this, I couldn't help but think about the fact that, while these insignificant characters were not the main heroes of the movie, they are who I should identify with most since, in the end of the day, I would be one of those people in such a crisis situation. This came off as very weird to me because it is so obvious who I am supposed to pay the most attention to in the movie, and yet I find that I [reasonably] emphasize most with these inconsequential characters; These small people who are really nothing in the grand scheme of the story.
6. Over the past few weeks, I've watched a lot of the marvel movies leading up to the avengers to make sure that the Avengers movie made the most sense possible. The strangest thing about the avengers was that each of the main characters was THE main character in a different movie. Suddenly I found myself ricocheting back and forth between the respective worlds of these different characters. I felt that the main point of this effect was to demonstrate that the ego feels that it is the main character of the world, the supreme hero. One sees the world from behind one's own eyes alone, and in a context where the most important thing in existence is ourselves and our own minds. To have seen the world from behind each of these characters' minds in their respective movies, and then to have these characters come together in The Avengers was an interesting parable of how we reconcile our solipsist/egoist tendencies with the existence of others. Egos clashed throughout the entire movie, making this the strongest point.
7. The portal that opens up in the second half of the movie looks unignorably like a giant tumultuous eye which, to me, symbolized the inevitable ego-destruction that occurs when the subjective and objective worlds meet.
8. Finally, the characters coming together with different skill sets and personality types symbolized, to me, how the different parts of one's mind come together to form a well-rounded personality. The fighting between the characters in the movie then symbolized internal struggle and how we deal with cognitive dissonance and personally difficult decisions.
That's all I can remember, for now, but I think I covered about all of the main ideas that I was overwhelmed by in the theatre. Sorry for any sloppy spelling and grammar. I typed this in a hurry.
Every day I am thankful that I was introduced to psychedelic drugs.