The game features more portions of the story and themes of MGSV that involve Racism and Revenge. The game play has evolved to be open world with a lot of freedom of approach with realism of scale, scope, weather and day/night cycles. Improved CQC that lets you take enemy weapons and use it against them, vehicles and horseback riding, and plenty more stealth mechanics! While we're still confused as to what the difference is between 'MGSV: The Phantom Pain' and 'MGS: Ground Zeroes', the latest information seems to be that Ground Zeroes is still intended as a prologue intended to release before MGSV. The reasons are perhaps that MGSV will still take awhile to release, so Konami intends to release a chunk of the game early to make the wait more sustainable. Hideo Kojima hopes it'll help ease players into the changed mechanics and open world design of Metal Gear ahead of MGSV which will be a much much bigger affair.
Also announced earlier during Konami's pre-E3 press video was the fact that a new voice actor has been chosen to play the ageing Big Boss, and it is Kiefer Sutherland! The official reasoning behind this decision and dropping long time series voice actor David Hayter, appears to be because Hideo Kojima wishes to have a mroe expressive and talented actor performing the facial motion capture. Many fans, undoubtedly upset by the decision are also theorizing that there's more to it than that and that in some form or another David Hayter might still be involved, maybe in the future as a return of a younger Solid Snake. But who knows? You can check out the Metal Gear segment from Konami's video below.