Pierce has always had the defense in him, but he expended so much energy on offense that he often simply was not used as a primary defender -- save energy and fouls because he had to be out there. This year, with more offensive options, the ball was in his hands less on offense, and the system demanded less of him on that side of the floor. Different areas of emphasis, and then this team's DESIRE to defend from day 1 drove him as much as anybody. I mean Ray Allen, a notorious turnstyle played excellent bad defense (if you are a bad defender, you can play excellent bad defense by steering a guy to help -- team defense as it were).
If he is not asked to be the primary guy on every offensive set, his energy is well managed and he can be used to influence the game defensively as well.