Josh Stoddard’s review published on Letterboxd :
Almost forgot this is called Avengers: Assemble here in the UK until the DVD I've never used reminded me. If anything, works better with the sequel titles than the real one does.
Fell asleep about 30 minutes in - I've been passing out super early for a ridiculous number of hours three days running now. Nevertheless, it means I watched the whole first act. Pretty sure I've seen people criticise this retrospectively as slow and boring but to me, it's perfect set-up for the plot and introduction to the main players. It might be cheesy, but I like how the last line of a scene tells you who's coming. Even now, it's exciting for the Avengers to make their totally-in-character entrances.
In fact, the entire structure of this movie reminded me of a Lessons from the Screenplay video I recommend watching, and lead me to some more readings, an epiphany and ultimately, helped my final project for my screenwriting Master's.
So, yeah, Joss Whedon might generally be a bad bloke, but he structures his scripts well.