If any of you have been following the development of Windows 8, you might be aware they are integrating the Xbox services into the OS. Maybe, the delay in PC announcement is just because of this.
+ GameInformer said they demoed the game on PS3, which may well be in good hope that the PC version isn't a crappy port.
And a compilation from the GI reveal I gathered;
Planes/Bicycles/ATV/Jet Ski's
Map size is bigger than IV/SA/RDR combined
Underwater
Building rapelling system
Completely revamped Driving/Shooting/Melee systems and mechanics
Convertible cars
Return of Ammu-Nation
Detailed mini-games (Full fledged golf course)
Loads of things to spend your money on, a vibrant economy
Lots of detailed interiors, including skyscrapers
Dynamic pedestrians (Gardeners waiting at bus stops at 6AM, then you will see them gardening on properties by 9AM. Women shopping in supermarkets.)
Dynamic encounters (Random pedestrian events, ala RDR)