Owen Hughes’s review published on Letterboxd :
The very start of the film is excellent; Samuel L Jackson proving once again that he is Nick Fury. It's a nice introduction to a character that hasn't had a great deal of screen time in the films building up to this ultimate showdown. The action sequence following is a great set up to what is about to come, too.
After this initial fantastic start, Whedon slows the pace down slightly (and if I'm honest, spends just a little bit too long on this part) to re-introduce everybody. Black Widow, Agent Coulson, SHIELD, Captain America, Bruce Banner etc are all given time to make you familiar with them and to blow you away with how awesome they are.
There's then a slow, gentle build up to the real turning point of the film; the first appearance of Hulk. After that, the film REALLY builds momentum right the way through until the end. The story, the conflict, the tension, the effects, the action, the characters, the humour, the dialogue, the performances from the cast... it's pitch perfect. Faultless.
People will tell you that Hulk steals the film, but for me, Black Widow is the strongest character. That's not to say everyone else is useless, because they aren't. Every character is strong and given plenty of time to develop properly on screen, each with enough back story to make their place in the film interesting. Nobody felt shoe-horned in. Nobody was just making up numbers. Everyone from Agent Coulson, to Loki, to Iron Man was worthy of their place in the film. It's almost as if it was written by not just a true comic book fan, but as if it was written by someone who has a strong affinity with Marvel comics in particular! Hmmmm.
The film oozes cool. It is exactly what a Marvel comic book film should be. It was full of humour, not just in-jokes aimed at nerds like me (although it had a few extras thrown in), but things that anybody coming to this series of films completely fresh will be able to laugh at and instantly 'get'.
It had lots of really, really well shot action sequences. Probably trumping The Dark Knight in terms of comic-book-film action. Considering the huge cast, everybody was made to look tremendously exciting and integral.
And, to top it all off, it had a very well told and awesome story. It really excels itself in this respect. Of course, it's what you're expecting. A group of superheroes that have a bit of friction when trying to work together at first but in the end they come together and save the planet. But it's about the way it's told, the style that the film is plastered in, and the frankly impressive way that it combines the serious with the entertaining. It's far from a "leave your brain at the door" action film; it's clever, witty and will at the very least hold your attention for the duration.
The best word I think to describe this film is "exciting". Because the whole way through, it's exactly that. Even if you don't like comic book films, you'd be hard pressed to deny that this is a very well made, and very entertaining film. It has raised the bar for what a superhero film can be. The new Spider-Man and Dark Knight Rises films have just had their expectations kicked up a notch.