The game is great but yeah, the story isn't perfect, far from it.
Wonderful scenes here and here (Skull Face death, "We are Diamond Dogs.", etc.) but the ending was... I don't know what to say, really. But I guessed that we played a fake Big Boss, something felt wrong during the entire game and I saw that famous reflection in the helicopter, since then, I knew the end twist and when I saw it, I was like "Yeah, ok, what's next?" and after that... nothin'?
It sucks that the mission 51 was cut, and probably the Chapter 3 (don't know if this "Peace" screen is legit though).
And all that demon stuff... Where is it? The scene in the trailers where Venom screams in blood, in the burning village/whatever, it could have been great but nope, not in the game. Does he kill child soldiers? Nope, save them and we don't have the option to (not that I would like that but hey, Kojima promised me taboo stuff). When we kill our own staff? Don't have a choice and it's more depressive than anything else (but it's great).
Spec Ops: The Line is far better in this area, in that game, I felt bad for what I did and the evolution of the "hero" is really cool, I wanted to see something like this in Metal Gear Solid V: The Phantom Pain but... Oh well, I have a lot of fun anyway, the gameplay is just perfect for me but it's sad to see that the final Kojima's Metal Gear Solid ends like this.