Brokeback Mountain is a long short story, almost a novella in length. Its a good length and its nice to see someone use it, because people don't use that length very often.
The story is a mix of interesting strengths and weaknesses. The problems are that there are often some artificial sentences that seem forced to me. The scenes where the two cowboys talk about the relationship instead of just ignoring it don't work. They rang false to me whereas the rest of it was OK, but they would not have talked about that in that way. The inevitable self-loathing isn't really there very much and I doubt that Proulx got that part very well.
My main beef is that Proulx here is only writing about the stuff she gets and understands. She could have pushed harder here at the story. It is told in a safe way for a gay cowboy story. What is definitely here is a lot of potential. It is really quite a good story with a lot of mistakes in it. If Proulx could avoid those mistakes she could be a first rate, lasting writer. I'm definitely going to read more of her and see if some of her other work can be better. There is a lot of potential here.