David Lowery's The Green Knight is a wonderful movie, but it shouldn't be thought of as an adaptation of the poem Sir Gawain and the Green Knight. In saying that I am not criticizing it: it's a mistake to expect a movie that claims to be an adaptation of a novel or some other kind of written text to actually be an adaptation. It almost never is; it's often better to think of film adaptations as riffs on the originals. But Lowery's The Green Knight is not a riff on the poem so much as a photographic negative of it. 

Lowery's Round Table is a foreboding place in a cavernous dark hall — something like a brugh; Arthur’s court in the poem is a bright and airy environment of festivity and ease, less Heorot and more Les Très Riches Heures du Duc de Berry: 

The Gawain of the poem is the most accomplished and most fastidiously elegant member of that smart set, precisely the opposite of Lowery's Gawain, who is callow, self-indulgent, uncertain about his direction in life – basically a caricature of a millennial. Given the contrasting initial positions of the poem's protagonist and the movie's protagonist, their stories will necessarily trace very different arcs.

I think the arc followed by the movie is consistently interesting, and well presented. (Also, Dev Patel is fantastic.) The movie is visually glorious, often in ways that precisely and movingly illuminate Gawain's psychological state. However, it's the sort of story that we, today, are pretty comfortable with: a boy-man in failure-to-launch mode who finally finds something he believes in enough to bring him to the point of launching. The poem tells a very different kind of tale, one that we aren't nearly as interested in hearing: it's a story about how even the most celebrated and admired person in a particular society can be afflicted by the vices endemic to that society as a whole. And should that person ever come to the point of recognizing his own frailties and failings, which are to an even greater degree the frailties and failings of his society, then you can be sure that the society will make a point of not learning any of the lessons that he has so painfully learned. That's what the poem shows us. 

It's hard for me to imagine a film director today choosing to tell that one, though you might get a glimpse of what it would look like if you imagine an alternate final scene of The Social Network in which Mark Zuckerberg, instead of saying “I’m not a bad guy,” says, mournfully, “I’ve come to understand that I am a very bad guy” — only to have his confession laughed off.