In particular, check out Matt Cheney for a review that pretty much expresses my feelings about the film's quality, and Dan Hartland's discussion of how class affects the characters' actions, and then I'll just add my brief comment.
I like the fact that both the film and Proulx's original short story don't shy away and in fact stress the fact that, for all that their society and their upbringing had a very great deal to do with it, the main reason that Ennis and Jack don't end up together is Ennis and Jack. This isn't to dismiss the no doubt tremendous, perhaps insurmountable, obstacles that two gay men living together in 1960s Wyoming would have had to face, but the fact remains that Ennis and Jack didn't even try. They walked away from each after their time on Brokeback Mountain, and in so doing essentially sealed their fates--everything that happens later is a result of that moment when neither one of them had the courage or the self-awareness to say 'this is it and I'm not letting go of it'.
I'm not saying this to be critical of either the story or the characters. The best tragic love stories aren't the ones where lovers are torn apart by uncontrollable circumstances but the ones where love fails because of ordinary, flawed humanity--Newland Archer refusing to start a new life with Ellen Olenska because a lifetime in New York society has calcified his soul. It's the same with Brokeback Mountain, and for all the marketing hoopla about the film being a romantic epic about a love that transcends time and distance (did the marketing wonks even watch the film, I wonder), the film itself is a tragedy about human nature and the simple fact that, no matter what the storybooks tell us, love isn't stronger than anything. Frailty, poverty, and time will wear away at it, and at us. Without ignoring the fact that they were carefully made as they are by social and economic forces, the film very definitely blames Ennis and Jack for ending up as they do and failing to achieve happiness.
All of which is fine, until we start to notice the other heartbreaks that Ennis and Jack are blamed for--the damage they do to their wives and children. While there's no question that Alma and Lureen suffer because of their husbands' choices (one of the ways in which the film improves on Proulx's story is the scene in which Alma--remarried, pregnant, and comfortably well-off--finally confronts Ennis about the truth of his relationship with Jack. Where the story made that confrontation seem waspish, an attempt to hurt Ennis from a position of strength, the film makes it clear that Alma has in no way recovered from the heartbreak that Ennis caused her. She is finally expressing her rage and betrayal at his actions--and Ennis doesn't see either), Lee's Brokeback Mountain seems to suggest that Ennis and Jack's failures as husbands and fathers are just as important, if not more so, than their failures as lovers. Which strikes me as a distortion of the original story's message.
It's true that Ennis and Jack can't seem to conceive of women as being capable of the same emotions as they are, and that they can't connect the love that they feel for one another with the kind of love they owe their wives. Although Ennis threatens to kill Jack for sleeping with another man, the two joke and tease one another over their relationships with women--in the clear understanding that these relationships are in no way meaningful. The addition of Cassie, Ennis' girlfriend after his divorce from Alma and one of the very few major deviations from the original text, is clearly meant to address this issue. "Girls don't fall in love with fun!" she exclaims at Ennis after he drops her without a word, finally clueing Ennis (who has recently had a painful and, as he doesn't yet know, final encounter with Jack in which they both railed at the pain that their love was causing them) in on the fact that the love that women feel is just as painful, just as complicated and just as thorny as the love that men feel. When his grown daughter comes to see him in the film's coda and tells him that she's getting married (instead of already being married as she is in the story), Ennis emotionally asks her if her future husband loves her, clearly thinking of the pain he and Jack caused their own wives by not loving them enough.
Which, to my mind, comes dangerously close to suggesting that Ennis had to experience the loss of Jack so that he'd learn to be more understanding and loving to the women in his life. There's no question that Alma, Lureen, and their children deserve our pity--perhaps more even than Ennis and Jack do, as their suffering was caused by no choice of theirs beyond the choice to love, trust, and marry an untrustworthy man, but Brokeback Mountain was never intended as a tragedy about being the wife or the child of a closeted gay man. While I certainly can't complain about a film that shows a repressed, emotionally unconnected man coming to understand and respect women to a greater degree, I don't think it's too much to expect that a tragic love story between two men (or two people of any gender), should focus on the lovers and not on the effect they have on the secondary characters.