For me it was the bit in the Petrified Forest. You know, with the wheelbarrow. Before GF, the only adventure games I had played were children's adventures in which every puzzle was along the lines of "take this item to characer A, he'll give you another item you have to take to B etc." so the idea of a puzzle that didn't involve the inventory was all new to me.
Aww, I was such a newbie.