I think, for me, this scene is the most visceral in the whole movie. I didn't realize it, when I saw it with my husband, that my hand was gripping his bicep so hard, he literally had to reach over and pry my fingers off him.
I think what shows so much is Jack, waiting for Ennis--he's already taken his shirt off in the cold Wyoming mountains; he knows what he wants and he wants Ennis to know. Ennis is hesitant, but I loved how he took off his hat and held it when he came in the tent-- a sign of respect and yes,of courting. (He'd take off his hat before entering a lady's house,right?) Jack comforts him(a sign everyone seems to have noticed), by telling him it's alright, and then, when they curl up together and Ennis is touching Jack's chest and face, really look at Ennis's face. He looks like he's just about to burst into tears-- like he realizes this is the best thing that has ever happened to him, even though he can't quite accept it (hence the keeping-his-eyes-closed thing).
So romantic, so loving. I wish that scene had gone one much longer-- hey Ang, we're anxiously awaiting the director's cut on dvd!!!