Easily the best film of 1971
Here's why. To me, this film constitutes a crucial link in the trajectory of films that broke the genre limitations of the western. And that link is what connects Once Upon a Time in the West (1968) with Unforgiven (1992).
*********************SOME SPOILERS BELOW******************************
In Altman's film, just as in Leone's, the main protagonist starts out as a larger-than-life mythical figure. The tavern folk treat McCabe with respect, trading rumors about his glorious past, refer to him as a "gun-slinger." The way in which McCabe is filmed as he approaches the tavern, complete with the sequence where he lights a cigar before crossing the bridge, everything in the opening scenes paints him as a mythical larger-than-life hero.
Now Leone never demythologized Harmonica. He alluded to the fact that Harmonica, just like Cheyenne, eventually would die, and the camera, instead of showing him simply ride off (cf. Good Bad and Ugly) instead turns to the sky. Harmonica, Cheyenne, and Frank all are the last of the great heroes of the past, and the film is a farewell to those figures painted against the backdrop of the emerging new West.
Altman entirely demythologizes the main protagonist cliche. We learn very soon that McCabe is not a gun-slinger, rather he is a savvy businessman, the kind of guy who builds his business using his brain, and to a certain extent his image. But the thing is we never see just how he got to build his image. Worse yet, we don't even really see any glimpses of the man he is rumored to be. Has he shot anybody at all, ever? We never find out.
On to Constance. At first she is just a mirror reflection of Leone's Jill McBain. Both are whores and really don't mind it, these are the kind of women who really helped build the West. The difference creeps in as the story unravels, through subtle clues. Constance smokes something, maybe opium, maybe something else. We know that she does it in order to get away from the reality. That's the main difference between her and Jill McBain, who really wasn't endowed with an existentially multi-dimensional perspective. Just like McBain, Constance understands what life really is all about. Unlike Jill, she feels the urge to escape from it. The pipe does the trick.
So the question is this, then. Just what does Constance know about life that depresses her so much? It is in the realm of this existential question, in my opinion, that Altman truly expands on Leone's marvelous draft, making a sequel to that movie as it were.
*************************MAJOR SPOILERS BELOW*****************************
I saw McCabe and Constance as representative of the two main character traits that helped build the West. He stood for the idealism, she was an embodiment of the pragmatism. Over the course of the film, McCabe believes that he can find his out of any situation relying merely on his smarts. Even when the assassins hit the town, he still thinks that negotiation is an option. Conversely, long before the trio shows up Constance already knows what is going to happen. Another example. McCabe keeps hoping that one of these days Constance will give him her body for free. He believes in emotions having a higher value than money, in other words. Constance in the meantime keeps charging everyone including McCabe, because she knows that nothing is more valuable than money.
But it's killing her, you see. She smokes the pipe to get away from this reality for a while. There is nothing else that she can do.
Note how the final confrontation between McCabe and the assassins takes place in the quiet deserted streets of the town. Everybody is at first asleep, then gone to the outskirts to put out the fire. Just as the heroes in Leone's masterpiece, Harmonica and Cheyenne, McCabe really is way past his time. The era of the small man who built the country is fading (note what everybody in the town is doing in the meantime, putting out a fire=building) and the large corporations are beginning to take over. Remember how at the end after he shoots the last assassin he kicks the guy's rifle so as to get it to be pointed to the ground? Now there is no other explanation to that move other than McCabe is simply making sure the rifle is not going to go off in his face. In other words, even with two bullets in him, he still believes that he can, and that he in fact has, survived the challenge.
The film consitutes the next logical step after OUTW because McCabe is not a gun-slinger in the first place, something Harmonica and Cheyenne were. And McCabe's opposition is not a mythical bad boy in a dark hat, but a faceless corporation. After those heroes faded, the McCabe of the West came in. Now it is his turn to fade, too. The whore remains. But she wishes she weren't there, either.
Unforgiven was really special because it actually brought the larger-than-life heroes back for one last go-around. But it brought them back old, cranky, and disturbed. Unforgiven was really the last great western, following McCabe and OUTW before it.