It is honestly best you watch it yourself, as his way of explaining it is really in-depth and discusses lots of particular points that click, but in general:
1. Greg felt that GZ was a shoebox. He says TPP gets rid of this completely. Each segment for infiltration is a "concept". You are given a scenario with many "real" points of entry with many valid ways to approach it. This MG does a great job of making the concepts of GZ something totally different in this way. He goes more in-depth on this in the video.
2. The game felt like it "was with you" no matter the decisions you make. No longer is the game like GZ in this regard, according to him. Stealth is still recc. for that easier S rank, but with the addition of 'bonus conditions' you can still get that S in lethal mode. This stops the game from being repetitive. No longer will you feel like you have to get that tranq the whole way through in order to "win". Lethal options are very valid means of winning the mission, and using lethal weapons and paths are some times necessary in order to do a mission without getting frustrated. And once again, the game doesn't make you feel guilty or that you lost by choosing a lethal way, though it does not give you as many points as possible.
3. There are mechanics in the game that you can potentially get to in 20-25 hours of playing the game. All these new mechanics are steadily introduced in the game and will keep you entertained.
4. Besides playing missions over for the "s" rank, which you might be doing later on in the game, there are still well over 100 side ops that are all their own thing.
5. Re-iterated that weapon customization really makes the game a lot more fun.
6. Though there is two large maps, don't worry. There are still variety in that open world including some pretty interesting infrastructures.
etc.
I really liked his points. It takes watching the whole video to just understand where he is coming from.