ALL those criticisms are annoying -- but if I had to pick I think I'd say the most annoying was the one that whines that it was "slow" and "boring". The entirle first sequence on the mountain, as they were getting to know each other, was paced exactly right for rural Wyoming in the '60s. It was beautiful, and idyllic, and almost like a dream -- before they came back down the mountain to dreary reality.
Whenever someone calls it "too slow", I always know it's some airhead who needs flashing lights and loud noises every five minutes or his/her poor little brain slips into neutral from lack of stimulation. People like that are going to have a lot of trouble in the real world, where there aren't usually exciting car chases to liven things up.
About adultery, I saved a file with a LONG list of movies and famous literature where adultery was a main theme, if not the entire point, but people didn't constantly obsess about their being unfaithful in them. That's just an excuse to condemn it without being blatantly homophobic. And actually, I disliked Alma a lot, I think partly because she was intruding on Ennis's relationship with Jack, making him cheat on HIM.
And the "too fast" criticism is usually levelled by people who just weren't paying attention, for the first half hour, to all the subtle details of flirting and teasing and falling in love. Some people have to have things spelled out for them. And I agree with the opinion posted above that nobody seems to think Jack and Lureen's affair came out of nowhere when they had known each other for mere HOURS, not months. Give me a break.