I've got to be honest here... every single scene before New York was really uninteresting and run of the mill stuff. The only real way to describe the first two thirds of the movie is "watchable".
The last third, however... was incredible. This movie is just overall quite a stupid achievement. The plot is ridiculously vanilla and lazy, there's no drive behind it apart from "here are these 4 loose cannons, see how fun they can be to watch?", Loki is a lame excuse for a villain (even though he is a great character) and fails miserably to provide any feeling of threat, plus the action in the majority of the film is just unambitious and condescending towards it's audience. Add one of the worst cold opens of all time and you have a sure recipe for disaster.
And then we enter the third act. Everything remains just as dumb an unfocused as it's been from the start, but the action... is just perfect, for lack of a better word. Hulk's smashing endeavours in particular shine as bright as blockbuster action has ever shone.
So yeah, I guess the movie is redeemed. The movie is a convoluted mess saved by a single grace, which is one of the best action sequences in recent history. Does that mean it's worth watching? Absolutely. But that hardly means it deserves the insane amount of praise it's getting. It's bloated and hollow.