Microsoft will add brand new anti-piracy measures to Games for Windows - Live, plus an in-game Marketplace and global game-saves that will be accessible from any computer.
The anti-piracy measures specifically target zero-day cracks, which make games available for download before retail launch. Microsoft will punch this in the face by requiring authentication for online play.
The in-game Marketplace, on the other hand, lets developers create unique store-fronts for their additional content, a bit like the Rock Band and SingStar stores.
Finally, the Roaming saves ties your progress to your Games for Windows - Live account, which you can sign into wherever you can find a PC with an internet connection.
Microsoft GfW - Live boss Ron Pessner reckons this is the "tipping point" for the service as an online PC gaming network. There's no word when that tipping will be done, however.