The western is a genre known for the unwavering codes and morals of their protagonists. Heroes will do anything to defend their town from the tyranny of an outsider. Law, order, and honour are all at the heart of the society they inhabit, and nothing will force them to reconsider this strict ethical code.
McCabe and Mrs Miller has certainly got the setting of a Western: a new frontier town which has quickly expanded and is still finding its feet when a stranger (McCabe) comes in and changes everything. However, he's not like the heroes of traditional westerns: he's a gambler- an opportunist, who sees a way of making a life for himself in this new town.
Part of the deal involves giving the townspeople (exclusively men) what they want (women): so he sets up three tents for some ladies of the night to service the males of the town. Mrs Miller, an inhabitant of a nearby settlement, hears about this, and convinces McCabe to expand this part of his empire, and so they become business partners in the town of Presbyterian Church.
The movie has a very 'modern' feeling to it. The way the central character is neither good nor evil, neither heroic nor especially cowardly and neither especially able or a complete wash-out allows the film to continue to have relevance to audiences today. In a way other movies of the Western genre, while enjoyable as entertainment, fall short on social commentary/relevance.
In particular, its critique of capitalism (around half way through the movie, McCable is confronted with a 'hostile takeover' - literally) would certainly ring true with the fans of The Wire. As success brings him to the attention of outside forces who make him an offer he can't refuse.
Perhaps its only weakness is that its 'radical' take on prostitution feels so idealistic in comparison to the more sobering themes of the rest of the film. Once it's taken over by Mrs. Miller, the whore house is presented more like a sorority house: with the women empowered by their ability to bring pleasure to the men of the town. They giggle, they chuckle, they dance, they eat cake. This is very much a film of the free love generation. Compare it to other depictions of prostitution in Slow Motion or Requiem for a Dream where clients pay for a service and can essentially demand anything in return, and you can't help but wonder if the director has really thought this scenario through.
So overall, McCabe and Mrs. Miller is a very enjoyable and watchable movie with a central character that's incredibly engaging and beautifully ambiguous. A Western then, for people who hate Westerns, an old movie for a modern audience.