The acquisition of Hollis Thomas. Based on his experience and the ability to play well in the middle shut down the run game of most teams. The inability for teams to run, allowed Peppers to be more effective on the corners.
I think it would be interesting on your takes as to what was truly the turning point of the season for the Panthers. Clearly their record of late allowed John Fox to continue his operation directing the team, and Marty Hurney his ability to acquire talent.