Playing the 360 version.
The Prologue is long winded but its like 30-40 minutes, after that there's almost no training wheels for gameplay stuff besides the really quick motherbase tutorial after rescuing Kaz. Gameplay wise after the prologue, this shit is pretty dope. A lot of people moaned about this not having MGS styled gameplay, but the main missions and free roam stuff feels like constant infiltration, you feel like an agent on the move constantly and getting shit done. You're doing shit Big Boss and Solid Snake would do outside their known story beats.
People have criticized the open world aspects being too empty, but the extra space helps with planning tenfold. You're able to peruse the perimeter without getting your ass hounded by apeshit guards. And I don't feel like traversal is that much of a chore compared to a other open world games, the open world itself isn't that large either. Big and spacious when it comes to stealthing around, but not so big that it feels like you're doing nothing for long periods of time.
I believe any main missions can be replayed after you first complete them. Side ops can't be replayed. Missions are meant to be replayed to get higher grades, you barely have any equipment at the start of the game. I'm playing with most visual aides off and reflex mode off, not killing dudes, getting B on the main missions thus far.
In short, a pretty dope game with a long winded intro but shit opens up quick.