From a technical standpoint, it's an open-world that has as much verticallity as horizontality. Most if not all open-world games lock you to ground level, even games with skyscrapers like Grand Theft Auto. Because BotW allows you to climb nearly every surface, the world is designed to invite you to do so, with mountains, cliffs, and canyons. It's the climbing mechanic that knocks the doors off of the exploration in a way that no other open-world game has yet.
Aesthetically this is the first cartoony free-roam open-world game since... I don't know, Jak2? Wind Waker? That's already something special. Beyond that the world has this calm, Ghibli-esque charm to it. Just running through a grassy field while soft piano music plays is a wonderous sensation.
And personally it just feels like pure exploration, without any sort of loot or upgrade or EXP system clouding it. If I saw a hill I went to the top of it just for the view. If from there I spotted a little forrested area, I would go to it just to wander around. I'd stand on a cliff or the beach just to watch the sun rise. I know, sounds gay and not very eventful, but BotW knows how to make being a simple wanderer enjoying the scenery compelling as all hell.