Actually Natal will use a full RGB motion capture camera. The video is fairly impressive and I believe should function rather well as long as it has the processing power, and from the looks of just the add-on unit it should have lol.
The FB and Twitter integration will be nice especially for me since I never check my accounts anyway. The direct 2 drive game system may be "OK" but unless there is a substantial discount for the games I would rather buy the physical copy because it provides you with more leverage later on when you don't want it, however a digital copy on your HD will not and you may unfortunately be stuck with something you don't want.