I just finished reading Anne Proulx's short story (LOVED IT). I saw BBM (just once) about 2 weeks ago, so from my perspective, the short story is a nice "additive" or "secondary source" to the movie. It helps round out the full story, especially in regards to some dialogue that was skipped in the movie. I say this with all due respect to Anne and her story. It's just that the movie is SO affecting and SO moving to me, it is the heart and soul of the story.
In regards to the dream sequences, I am actually glad they were left out. As mentioned by others, I think an ending dream sequence would have been unnecessary, for at movie's end, we all know Ennis is, was, and forever will be thinking about Jack, whether awake or dreaming.
In regards to an opening dream sequence, I appreciated the movie even more because I simply did not know what was going to happen. I mean, before entering the theatre, I knew that the two boys would get it on and fall in love. But I had NO WAY of knowing that Jack was going to die, and that there would be so much left unsaid, undone, and that by movie's end I would be heartbroken. By not portending Jack's death (at least I for one did not see it coming), the movie became even MORE POWERFUL to me...