Not really one thing. The fact that it's pretty much Peace Walker 2. But also:
Fulton.
Every base building aspect from PW being in the game even mini games like Outer Ops, but more advanced and better implemented.
The incredible gameplay from GZ in the huge world of TPP.
The open world. You get so much freedom in the way you can approach missions. Most other open world games have a huge map where you can get lost in but the missions are always very linear. MGSV's approach is so much better, you get so much more freedom. You can be very creative and finish missions in so many different ways. GZ proved that already, just like at this video for example:
https://www.youtube.com/watch?v=yDHVwCSMn1A
And that's just one mission. TPP is going to be 100 times larger and will have hundreds of Camp Omega's and a wide variety of different missions and locations.
Alot of different vehicles for transportation: choppers, jeeps, tanks, horse, motorcycle
and even a mini Metal Gear.
The tapes. I loved them in Peace Walker, they add alot of backstory. And now we can listen to them while playing the game, and we can even collect 80's music.
All the little game details that we got in recent impressions, it's just too much to list.
And obviously the story, going full circle. BB's descent into madness, with alot of inspiration from books like 1984 and Moby Dick + Hideo Kojima's writing=