I always feel very moved by that look, and by the thought that this is the first time he's experienced this.
Me too. When people are assessing Heath's performance, they should think not just about the big dramatic scenes but also about tiny moments like this. Somehow, very subtly, he conveys what this conversation means to Ennis without ever doing anything overt or obvious to telegraph it. For instance, some actors (and some directors) would show that this is a novel experience for Ennis by have Ennis gaze at Jack with amazement and admiration. Here, we most understand how Ennis is feeling when he's NOT looking at Jack, like at this moment or when he fumbles for a cigarette.
BTW, I remember once we were discussing what facial expressions from BBM we found ourselves adapting, and someone mentioned this one. I was pretty impressed; it's a hard one to duplicate.