I was thinking that Jack's discovery of fresh water in the caves leads to an interesting moment in the series design of Lost. That's the point where the survivors factionalise into 2 groups. The first stay on the beach, refusing to give up hope that they'll be discovered. The second group follow Jack; they want to focus on the necessities of survival.
I talked earlier about the survival plot. That where I imagine the writers figured out, "In this episode they'll have to deal with finding water. In this ep, they'll have to find food." There's also been the social plot - which asks how group dynamics work. When you meet 40 strangers, how do you establish leadership? Resolve conflicts?
The discovery of water away from the beach marks the first point where survival impacts on social. I certainly wouldn't have expected the group to factionalise that early. In a way, this marks a moment where the show goes from being about just anybody being on the island to the choices that this very specific group of survivors have to make.