It's really tough to explain!
For a start, the delivering aspect is the game. From what I can tell, there's not really more to it than that. There's a promise of weapons and combat later on, and from what I've heard, you end the game with a bunch of guns, but it's still deliveries from start to finish. But there's something about it that's just incredibly satisfying. It looks absolutely stunning, and it feels great to play. The movement and the moment-to-moment gameplay feels very reminiscent of MGSV (just without the combat). You're constantly being made aware of how much you're carrying and the effect it has on you, so it becomes very much about managing what you've got, why you've got it and where you're taking it. There's a lot of very minor decisions you have to make, but they all feel significant, and having to choose one thing over the other genuinely feels like it's going to have a knock on effect.
It's certainly not for everyone, and in a way it is sort of boring, but that slow nature just makes it all the more tense and suspenseful when something does happen. Earlier on I was carrying some cargo and I started getting chased by some mules (they want to steal your cargo to sell on for themselves), and it very much became about getting away quickly, and due to that it became about getting rid of certain items to ensure I could out-pace them. It was the same sort of stress that came from trying to sneak around an enemy camp in MGSV, and it felt wonderful. Not to mention the BT's, who are genuinely unsettling and having to manage cargo, plus your breath, plus your balance, plus your speed, plus your need to get away was just fantastic.
The story so far is incomprehensible in the best possible way. It tells you nothing, but Kojima can world-build like no one else. Everything feels real, I believe it all whole-heartedly and the characters are especially compelling, and I'm genuienly invested in what happens to all of them.
This game is marmite, you're going to either love it or hate it. If you're on the fence, ask yourself whether you want a game where Hideo Kojima got handed a blank check and got told to make whatever it is he wanted to make without any filter or direction from above. Because that's exactly what this is. This is a game that should have been made by a tiny studio of one or two people with a £20k kickstarter budget. Instead it's the biggest release of the year. It shouldn't exist, but it does, and that makes it all the better.