Activision-Blizzard have declared that they want to 'limit the supply' of pre-owned titles by increasing the importance of DLC in games. Basically, they want the disc you pass on to your friends to be worth a lot less than the full experience you get when you buy the game new.
2010 has already seen EA Sports implement a one-use only activation pass to unlock important content. This scuppers a lot of potential second-hand trading of its titles because it devalues the game for second users. Ubisoft have publicly supported the idea and THQ have emulated EA's activation scheme.
Activision-Blizzard's CFO Thomas Tippl said, "We are still evaluating various possibilities for greater participation in the used-games business. What's been working the best so far is providing additional content and therefore limiting the supply to used games."