Some missions will apparently have time limits, among other things.
“With MGS up to now, we could only build the interior of wherever you were infiltrating. How you got there was shown in a cut-scene, and the player would just suddenly be in front of the entrance. Once you finished the mission, there’d be another cut-scene, 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 it’s not so much an open world title as just giving you more freedom in how you choose to complete your missions and more side-missions to contend with. Metal Gear Solid 5: The Phantom Pain is slated to release on Xbox One, Xbox 360, PS3 and PS4.