First of all, I apologized if there is any mistake, my english isn't perfect.
Just finished the prologue and first chapter. Everything was top tier, but I would've loved it WAAAY more if I hadn't seen all of the trailers.
Gameplay is amazing, it's ground zeroes but better. I honestly think this could become one of the best game I've played (gameplay wise).
Personally, I think that big boss talks less because of those 9 years in coma, you are a new man, you are not the person you used to be, you are being born again (If you played the prologue you shoulod know where this is coming from). Big Boss literally asks miller to tell him what to do next like he used to. The main topic of the game seems to be the loss of the tongue/language. Maybe the decision to not use David Hayter has something to do with this, maybe it was needed to show this change in Big Boss persona.
Please, if anyone answer to this, remember I've just finished the first chapter, so no spoilers from anything beyond that please.
That's how I interpreted it