Do you remember when armor penetration was all the rage? While combat rogues lusted after all the ArP as they could get their hands on, the other physical DPS classes shared the same desire. I forgave all of the feral druids who asked for the leather gear with armor penetration on it; I know we have to share the agility-based leather gear with our furry friends. I didn't put my foot down until hunters, warriors, and even death knights started rolling on our stuff. While having such a powerful stat allowed us to scale competitively, it also created a massive amount of gear competition.
Blizzard's developers decided to end armor penetration's reign as the top stat and actually named a successor to the throne -- agility. Agility is now a rogue's best stat, and since it's always on our gear, we never have to worry about whether we'll see it or not. The plate DPS classes now only want strength, and so our precious leather is safe from their clutches. The stat redesign also shifted the balance of power between the other secondary stats, and we saw several stat functions changed as well.