Awesome write up. As I've said before, Avengers is one of the three comic book movies I place into my "God Tier" alongside The Dark Knight and Spider-Man 2, and I don't say that lightly. I think the film is an absolute masterpiece, an absolute JOY to watch and rewatch, and still the best MCU film (though, having seen Civil War once, that might change after a couple more watches!)
I don't have much to add that the OP hasn't already covered or this great video:
touches upon, but I'd like to mention a few things.
As the OP went over, I loved the introduction to the film, and then the introduction of each character. This is the first thing of many that Age of Ultron COMPLETELY fails at. Like Whedon forgot what made the first film great. In AoU, the Avengers are just... assembled and attacking a snow base? Why? What exactly about that situation necessitated the fucking Avengers? Cap could have probably handled that situation by himself with ease.
Now look at the Avengers. Mother fucking Loki (who has already been established in Thor, but whether you saw that film or not it really doesn't matter) comes in and steals the tesseract from Samuel L Jackson, creating a wormhole that fucking destroys a massive SHIELD building. fuuuuuuuuuuuck. there hasn't been a better MCU intro scene since, nor a better MCU villain.
and then you get this scene:
"As of right now, we are at war."
"What do we do?"
BOOOOOOOOOOM
And then the music hits.
Fucking perfection right there. The hype and excitement hits a high there and continues for the rest of the film. Shit is about to get really real.
The writing and humour in the film is great. You have your expected quips and lines from Robert Downey Jr,
but also now classic lines from Cap (I understood that reference!) and Thor (He's adopted...)
We also get the most memorable fight scenes in the MCU (until you all see Civil War!) with the awesome fight in the park, the fight on the helicarrier and the fight in NYC. If there's one word I could use to describe The Avengers, it's memorable.
You have this:
and this:
You have the perfect "theatre cheering" moment (to this day the loudest reaction to a film I've ever heard in a theatre. the place ERUPTED for at least 10 seconds and we couldn't even hear the "puny god" line):
And you have the perfect payoff to not only a single film, but multiple films:
ahhhhhhhh soooooo good. It's all so good. There are few films like it that have managed to have me smiling throughout the entire thing.
compare that to age of ultron, where the only scenes I legitimately remember are... Cap almost lifting the hammer (HOLY SHIT) and vision lifting the hammer with ease (because FUCK THAT CAP DESERVED THAT MOMENT). quite a difference...
I also want to mention the music. While it's no Superman theme, or Elfman Batman theme, or Elfman Spider-Man theme...
The Avengers theme is probably my favorite and most memorable piece of music from a superhero movie in the modern era, alongside the Batman theme from the Nolan trilogy. I like it a lot, but it especially stands out in an era that really lacks awesome themes for their superhero movies.
Last but not least, you have the two scenes that has forced any comic book film fan to sit through the entire credits just in case something awesome happens, like...
HOLY SHIT
and...
lel
Fucking love this movie.