It's hard for me to separate this movie from the story, to talk about the qualities of the movie as separate from those of the book.
For me, the best and the worst of this movie lies in the music. The male choruses, often sung in Welsh, are truly beautiful. The story - there is no plot - is at best episodic. The only thing that gives it any coherence is the choral music, and that is very beautiful.
On the other hand, a lot of the background music is far too sentimental, and takes the movie over the edge. Had there been no music whatsoever, some of those scenes would have been far more powerful. The music, frankly, trivializes them.
The thing that most struck me with this movie, however, were the differences from Germinal, Zola's novel about miners not far from Wales, in northeastern France. Zola could find no nobility in their suffering; he depicted them over and over as bestial animals. This movie gives their suffering a nobility that no amount of sentimental music can lessen.
Men and women such as these are not perfect. Who among us is? But their suffering deserves respectful treatment. Zola did not believe that; John Ford, the director of this movie, did. The difference is telling, and it is what makes this such a moving movie.
Postscript written after having read the novel:
I read the novel after writing the above review and then watched the movie again. Frankly, I don't recommend that to anyone. The novel is long, 500 pages, and the movie, of necessity, condenses some of it and simply omits other parts. After you have read the novel, you see just how hastily some things are condensed and what is left out or, in some few instances, changed. I would not say it is a particularly good condensation.
The most striking change is the end. In the movie there is another mine cave-in and a party of miners, led by the preacher, finds the elder Morgan, who dies in the still very young Huw's arms. It is, of course, moving, but since the movie closes on the images from his past that Huw subsequently recollects we are left, at the end, not with the dying father but with Maureen O'Hara smiling and waiving at a smiling Walter Pidgeon. A romantic end.
The novel's end is far more somber. During a particularly bitter strike Huw's father goes down in the mines to see if they are starting to flood. Cyfartha is also down there working. Both men get caught in a cave-in. Dai Bando, almost blind, goes down with Huw to find them. (The preacher has by then left for South America and is no longer in the picture, so to speak.) After much digging they first find Cyfartha, whom Dai Bando, weeping, carries to the surface. (We are left to wonder if the very deep bond between these two men had been more than just a deep friendship, but the text never says more.) Then Huw finds his father, and holds him until he, too, dies. It is a very painful scene.
Other than that, the chronology is sometimes changed from the novel to the movie, prompted in part, perhaps, by the fact that since Huw is always played by the young Roddy McDowall, he never grows into the very adult man that Huw becomes in the novel. The two brothers who leave for America do so much earlier in the movie than in the novel, for example.
Huw's one sexual encounter, with Ceinwein up on the mountain, never takes place in the movie, perhaps because Roddy McDowall doesn't look old enough for sex, perhaps just for the morals code of the time. In the same sense, Huw's subsequent unrealized desires for Bron are never introduced.
The preacher never leaves for South America, probably because he is played by the lead actor. It is also hard to understand why the town condemns him and Angharad, as in the movie he does not visit her every evening at her home. More morals code, no doubt.
The disputes between the minors and the owners are played down in the movie, perhaps because, during the war, unions were expected not to strike "for the war effort."
Llewellyn's efforts to convey the Welsh his characters are speaking by using a similar syntax in English are almost completely erased in the movie. Indeed, the very fact that these characters are usually speaking Welsh and not English is largely ignored. It's a very important part of the novel and the world that Llewellyn described, which saw itself very much put upon by outsiders=Englishmen, so it is a shame all that is lost. Some of the Celtic folklore is also lost. Beth's claim that Ivor came to speak to her, very much a part of the Celtic belief in visitations from spirits of the dead (présignes, in French) is completely changed in the movie.
A movie should be not judged on how well it adapts the material from which it derives, unless it is some sort of documentary. Taken as is, this is a good movie. But once you have read the novel, you see that its story is more somber, and it is hard not to regret that some of that was lost in the conversion.