I honestly haven't watched that many westerns, and don't have anything personal against them. I phrased that poorly, as I meant that to be in reply to those that praise it for eschewing or mocking the stereotypical western traits by filling it with morally depraved characters and providing a grim setting. I'm looking for justification of its classic status, and so far the only thing I didn't pick up on was that the disfigured prostitute could have prevented a lot of trouble had she stuck up for the lesser assailant (in the darkness I thought he was helping the butcher). In not telling of the one cowboy's relative innocence, the message of the danger of false information was further supported. Otherwise, nothing is selling me.
What makes Unforgiven such a unique Western, and a unique film beyond just the genre, is the great texture that it gives the characters within. Nobody is a clear villain or a clear hero, and you can see their failures and flaws as they try to get through their lives. For his part, Eastwood's William Munny would rather forget his previous life, but gets dragged back into what should be a fairly straightforward job. The 'kid' finds out that he doesn't have the heart to do the deeds to truly earn what he started out wanting to be, a real gunslinger and killer. Gene Hackman's sheriff is a master manipulator, mostly of himself, as he truly believes that he's a good man, even while committing horrible and sadistic acts.
The photography and liberal use of natural lighting, the very dirty, brutal feel of the lifestyle and even the equipment and clothing in that enviroment shines through. This is no polished, re-imagined old West where everyone manages to be perfectly clean and wearing shiny pistols and brand-new bright shirts, with a bunch of people that look like they belong in a gap commercial. No, it's a hard scabble life, and it shows.
The music is understated, there is no rush to get from scene to scene, and we're often left lingering at the uncomfortable aftermath of a scene, seeing the visceral results of violence and cruelty visited upon each other.
Perhaps the ultimate achievement was the lack of redemption for William Munny, as befitting the title. He truly can't change who he is, and his transformation back into the ice cold murderer that he once was is simultaneously satisfying and a bit sad.
In short, it's a masterpiece.
BUT : it is slow moving, and doesn't have any grand shootout of the good guys vs. the bad guys, or anything like that. For those that tend towards films that have constant action and big explosions, I don't think it's a winner.
It's rewarding to me because I can examine the characters, and see their motives and conflicts within their emotions and choices, and the results are understandable. Too many times I'm watching a film and a character does something that is utterly stupid and doesn't fit the character, yet it's written in just due to expedience to the overall story.