This past week Wizards of the Coast ran an article about the core rules of Dungeons & Dragons. The current staffed were poled as to their thoughts on the rules and what rules had carried through the different editions of the game. The most constant point present was the power of the d20 to represent most actions. I have to agree, the d20 is what I associate with D&D more than any other die and this goes long beyond when the coined the d20 System.
What I noticed about the list was that no one listed role playing. My initial thoughts were, how bizarre or perhaps telling that role playing is not included in this list. Then I took a step back and I looked at the list and the initial request again. They were looking for rules, for mechanics. Role playing is neither of these things, it is a mindset.