"Infiltration and espionage." The cornerstone of the Metal Gear Solid franchise.
And according to what master Hideo Kojima told a French source (as cited by IncGamers ), Metal Gear Solid 5 will feature more in the way of both infiltration and espionage.
He also confirmed that the new game is running on the FOX engine and although he was intrigued by the idea of "social interaction" (as seen in last year's Deus Ex: Human Revolution ), he didn't say if something like that would be seen in MGS5. Oh, and those of you who are erroneously thinking that Metal Gear Rising: Revengeance is some sort of blight on the franchise, Kojima reminds you again that it isn't a Solid Snake game "and it was never pitched to the public as a Metal Gear Solid title. It's a spin-off, plain and simple.
Lastly, did you know Kojima wanted to kill off the legendary icon in MGS4? …that woulda sucked.