I want to defend the complexity of the story for a bit here. I would say that the movie brings the story to life, and that it fills in some of the gaps in a very spare story. But less complex? I would say that, for something so short, the story has a lot of complexity packed into it.
I agree that Jack's character is much more likeable without the implication that he'd been lying to Ennis about his relationships with other guys. Movie-Jack is a great deal more romantic of a character than story-Jack. Ennis, too, is more likeable, though I think that's as much because Heath Ledger just has such an amazing smile, so you can see the love trying to peek out from behind all of Ennis's inner conflicts.
But I think the story characters are complex as well. Jack is intriguing, because for most of the story I got the impression of him having sex with other guys, lying about that to Ennis, shooting eagles, so forth and so on. And then at the end the revelations start... the flashback to the dozy embrace, the twelve-hundred-mile drive for nothing, wanting his ashes scattered on Brokeback Mountain, and then to drive it all home, the pair of shirts. And suddenly I wanted to go back and try to figure out if there were any hints about the nature of this guy further back, to somehow reconcile the guy who told his father that Ennis was going to move up there and help lick the ranch into shape with the guy who claims not to have had sex with other men. I never quite managed to do it, and the movie gives us a different character, but still... I think the Jack of the story is a complex character in his own right.
And as for Ennis in the story... well, I read the story as mostly being Ennis's point of view (though there are some brief moments when we learn about Alma's or Jack's thoughts). And the strange contrasts in the language, the beautiful language used to describe the mountains and the coarse language used to talk about the men, seem to be part of the development of the character. It's as if Ennis has locked away the mushy and romantic parts of himself, and so we learn about this very romantic story in very unemotional language. And to me, that contrast, between the things going on in the story and the language used to describe them, is very powerful. (Also a fascinating literary technique; I learned about the story from a friend who thinks a lot about how writers use language for particular effects, and it seemed to me that the story illustrated a lot of the things she had been trying to explain to me.) So anyway, for me it was the very unemotional nature of the story that convinced me that Ennis's conflict was primarily internal, even if he spoke mostly about fears of physical attacks.*
(An additional note, about the looks of the guys. I don't generally picture characters when I read books, even if the author goes to great lengths to provide a physical description of them. And added to that, I didn't read the story in the New Yorker... I read it on-line back in October, and the web page had a promo picture of Heath and Jake playing Ennis and Jack. The guys weren't labeled, so I went through a couple readings of the story actually thinking that Heath was playing Jack and Jake was playing Ennis... Heath's got curlier hair in the pictures, and in the picture I had seen, Jake looked particularly lanky. I got the pair of them sorted out very quickly, when I went searching for more info about the movie. But I never pictured Jack with buck teeth... I always pictured both men as attractive, when I pictured them at all.
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* (Yeah, I know, Annie Proulx is mean to her characters in general, and doesn't seem interested in letting the reader empathize with them. Story-Ennis and story-Jack are about the most sympathetic of her characters that I've read. And Ang Lee tends to be gentler, to let the reader care about even very strange or unpleasant people, like the characters in
The Ice Storm, for instance. So some of those differences between the artists come out in the book versus the movie, I guess. But I still saw a core of characters I could care about in the story, even if lots of other people didn't see them.)
