Some anti-piracy measures have been added too with game licenses now linked to Gamertags, and 'zero day' protection which stops titles being played before official release.
A server-side authentication "links the game license to your Gamertag. The advantage of this method is that it allows installation of the game onto any PC, anywhere - only you can access the LIVE service from the game. Server Side Authentication has no affect on offline play."
Zero Day "keeps games from launching before the street date the publisher has set for the game." Your Live account can now be adjusted or otherwise tweaked while remaining in the game you're playing, which includes installing newly bought downloadable content apparently. Click to go grab the new Games for Windows Live 3.0 client.