Kojima talked about Metal Gear Solid V’s open world in an recent interview with Famitsu.
“With MGS up to now, we could only build the interior of wherever you were infiltrating. How you got there was shown in a cutscene, and the player would just suddenly be in front of the entrance. Once you finished the mission, there’d be another cutscene, a helicopter or whatever would come by, and you’d escape.”
“I think the term ‘open world’ has taken on a life of its own and caused misunderstandings. Of course it’s not going to be a game where Snake fishes all day or changes jobs and pursues a different life. The game map is an open world and you have freedom in that way, but in MGS5, it’s clear what you’re doing. That may be ‘I have to help someone’ or ‘Destroy this thing’ or ‘Go gather intelligence at this spot’. Some missions will have time limits, too.”
So the open world aspect is really about how you choose to approach situations, and it will be the player in control of every aspect of the mission, from infiltration to extraction. But you will still be given clear goals to accomplish.