Meh. Lower your expectations considerably. I would slot this as the lowest ranked movie in the "middle tier" of the Marvel Cinematic Universe. I give it a C+.
I don't see any way to explain why I think this without getting into SPOILERS -- so if you haven't seen it (and plan to), you should leave now.
Let me start with what's good. The movie is quite funny throughout, particularly in the first half as characters meet up with other characters they've never seen before. The script does an excellent job of retaining each hero's voice as established in the previous movies. Two aspects of this stood out in particular to me. First, there's a great oil-and-water quality to Doctor Strange and the rest of the characters he meets. Second, the Guardians of the Galaxy material retains those characters' quirky and irreverent vibe, just as surely as if James Gunn (writer and director of those two films) had overseen it.
Another impressive aspect is that nearly all of the characters get at least one important moment somewhere. That goes not just for the scads of "heroes," but even for many of the secondary characters who make more limited appearances -- people like Wong from Doctor Strange, Ned from Spider-Man, and Okoye and Shuri from Black Panther. This stuff most certainly had to be engineered backward in the writing, and the fact that the resulting story doesn't really show too many signs of this engineering is no mean feat.
Beyond that, I see a lot of flaws. Some are not too hard to overlook. The plot is awfully repetitive: Thanos threatens somebody's life to force someone else to give up information/a Stone/the Gauntlet. It's a bit of a waste to pair Iron Man with Spider-Man when we've already had a whole movie of that. Why leave out Hawkeye, one of the original six Avengers (according to the films, anyway)?
Then the flaws start to feel more serious to me. Despite all the engineering, they didn't engineer some of the dramatic character moments that seemed most desperately needed:
- Coming out of Captain America: Civil War, a reckoning between Tony and Steve seems like the single most emotionally resonant bit of character drama pending in the MCU. But they never see or say one word to each other in the entire movie.
- The last time Bruce Banner and Natasha Romanova were together, she'd finally opened her heart to someone after a lifetime of pushing people away, and he turned around and broke it. This is addressed with little more than a head nod, a "hey, s'up," and a cheap "awkwarrrrrd" joke from Falcon. (It's not awkward, actually, because they never confront the situation.)
- The last time this crew was all in one place, Captain America's actions indirectly led to War Machine being critically injured. Rhodey has a quick line or two to tell us he's made peace with that, but it feels wrong that someone with a conscience the size of Steve Rogers' wouldn't comment on it. And is there no part of Rhodey even a little reluctant to help Vision, given his role in what happened?
But my biggest issue with the story is that it's impossible to take all its "shocking deaths" seriously. When Loki and Heimdall both die in the opening, there's definitely a rush of "wow, we're really doing this." Thanos even says, "no resurrections this time." The stakes really are going to be larger than ever. But then, halfway through the movie, this sentiment starts to ring false.
It happened for me at the moment Gamora is killed. Part of the problem is that I thought she was totally right. Even with the flashback to Gamora's childhood earlier in the movie, I don't believe we've seen Thanos love anyone or anything enough to truly "sacrifice" it in a meaningful way. So the whole Soul Stone premise rang false to me. And once the skeptical mind is jump-started, it gets a lot to feast on in this movie. We know Guardians of the Galaxy, Vol. 3 is on the way. Are we really to believe that would happen without Gamora and Zoe Saldana?
This was just the appetizer for the main course to come once Thanos snaps his fingers. First, Bucky collapses in a heap of ash. Surprising to some, probably not to others -- but okay, one can imagine the MCU without him. And losing Bucky is certainly a gut punch for Captain America, who has spent a considerable amount of screen time trying to save his life. But then T'Challa dies? Bullshit. All the Guardians but Rocket? Bullshit. Spider-Man? Bull. Shit.
I acknowledge that you don't set up a Chekhov's Gun of a plot device like the Infinity Gauntlet and not expect Thanos to wipe out half the universe. But can we at least pretend like it might stick? They could even kill off important characters -- with no already-announced movies in the pipeline. We don't know, maybe for dramatic weight, they would kill Hulk or Thor (or even Iron Man or Captain America, if their outstanding business had been addressed). When it's all framed inside the narrative, as a few surviving heroes desperate to undo what's happened? That's heroic. When it's the corporate behemoth of Disney/Marvel, outside the narrative, making sure not to kill the cash cow? It's all a stupid gimmick.
In other words: no, Black Panther fans. They did not just kill the highest domestic grossing superhero of all time.
I suppose comic book fans are used to this death/rebirth stuff. But in this particular context, I think it renders the entire film devoid of meaning. If you're planning to roll back the clock, is there any logical place to draw the line? Did Gamora really die? Did Loki or Heimdall? How about any of the deaths in the MCU before this movie? Does any of it matter? Can any threat ever be taken seriously if even the most massive scale of annihilation imaginable isn't something anyone will have to live with?
Like Thanos himself, I'd round up half this movie and kill it to save the other half. The first chunk of Avengers: Infinity War is a fun thrill ride peppered with great jokes, skillfully mingling one of the largest cast of A-list actors ever assembled. The last half is muddled, noisy, empty, and deceptive.
So there you go... that's how I arrive at C+. Skimming social media, I suspect I am very much in the minority in this opinion. But it seems appropriate that this movie slots into my MCU list right under the original Iron Man. We really have come full circle, bookending 10 years of Marvel with two movies that most people seemed to like a whole lot more than I did.
P.S. -- I really wanted to call this blog post "Oh, Snap!" But that seemed like too big a spoiler.