1) License transferal - if you trade the game in, deactivate it and get something back. MS points. Who knows. But there should be some incentive to actually transfer the license to another gamer. The system should give the publisher (and by transferrance, the developer) some money. Frankly, I find Gamestop to be a horrible business model and never do business with them.
2) Lending the game - much like Amazon or Nook and books, loan the game to a friend. They get a limited time activation of the game on their account or there's a progress blocker at some point in the game's completion (or both, if there's a multi-player component). You should give them an adequate span to determine whether or not they want the game.