The biggest feature is the in-game marketplace, which lets you purchase items for your game from within the game itself. You no longer have to exit your game to view or purchase Game Add-ons.
Once you’ve bought something, Games for Windows – LIVE will download the Game Add-ons and install it all from within the game. You no longer have to leave the game experience to grab the latest addition to your favorite game! You can also re-download previous purchases if you need to as well.