The hype for the game comes because it has all of those things together, along with a full ecosystem of wildlife, machinelife, crafting using materials gained from the wildlife and the environment.
It's not just that this is a a huge open world game, but it's that, with real time, dynamic weather and real time lighting.
Weather will actually effect the environment that will have a knock on effect on the wildlife/machinelife, which will affect their behavior and thus that effects the gameplay.
Scale, systems in place for a reason and that will provide for massive gameplay potential, that's why I'm personally hyped and why I think so many others are, outside of the concept of a way far off future setting, with so much mystery surrounding the machines and the reasons for why the world is the way it is.
Seriously what western RPG has all of this together, with that kind of setting?
The biggest part comes down to how the gameplay seems to work and the fact you have to deal with huge robotic machines that look like animals, which many call robot Dinosaurs.
