Brokeback Mountain became iconic as a film, released in 2005. Before it was a movie, it was a short story written by Annie Proulx, published first in The New Yorker in 1997, later in a short story collection, and finally as an individual book.
The book tells the story of two "cowboys", who in 1963, develop a romantic and sexual attachment to each other that lasts throughout the years. Neither one considers themselves homosexual, and they both enter into heterosexual relationships, meeting once a year or so for sexual encounters. Although their backgrounds preclude any interest in, or even knowledge, of homosexuality, and their relationship is marked with a great deal of confusion, their attraction to each other overwhelms the taboos and difficulties of carrying on their affair.
Annie Proulx has a nice style of prose, although personally I found the most evocative passages ones describing the natural setting, while the description of either the main character's relationship, or of the social milieu they find themselves in, are rather sparse and/or depressing. Some of the passages describing the lovers' 'feelings of loss and lonesomeness are truly moving.
One of the problems with "Brokeback Mountain" is that it is impossible, at this point, to separate the movie and popular culture reach of the movie from the original short story. If this was just a short story I ran across by chance, I would probably consider it interesting and well done. But the real question is why this story became so famous.
And honestly, the answer is because it is about gay cowboys. If this story was about a cowboy who carried on a 20-year long affair with the waitress at the local diner, it would not be famous. Or if it was about two men who worked as computer programmers in San Jose who fell in love, it would not be famous. Even if Annie Proulx had other designs in mind, even if she was writing a story about love and loss, the cultural reception of the story comes from the fact that it is about taking an icon of masculinity, the American "cowboy", and flipping it around.
And the story is about icons. Call me crass and a sloppy reader, but after reading it, I couldn't remember which one was which. I couldn't point out any defining personality traits of Gay Cowboy #1 and Gay Cowboy #2, besides one was slightly more successful and settled than the other. And perhaps it is just my cynicism, but Annie Proulx fails to do what writers from Marcel Proust to Stephanie Meyer have failed to do: given the reader any idea of why the characters in the love story love each other. And while the story might be trying to humanize the characters by showing that stereotypical "tough guy" men have another side, it ends up doing just the opposite, because the characters become just icons, altered in one trait for reasons of cultural foiling.
So while I can't fault Annie Proulx, who seems to be a talented writer, for writing this story, I do think that it was so well received by the public for rather simplistic reasons.