For me, BOTW is the first open-worlder that doesn't feel like a "connect the dots" affair, going from waypoint to waypoint, sidequest to sidequest, collectible to collectible, looking at a map with hundreds of markers on it, etc. While at a very high level, you're doing those things, the manner in which you're doing them, and the freedom which you have to do it, feels different and so much more organic and natural than anything I've ever played. I also like that the game more or less gives you everything you need to solve virtually every puzzle in the game within the first few hours, so it's essentially you vs. the world from that point forward instead of always focusing on what you need to do next to gain access to the next tool. That's a huge, huge change for Zelda, and it also makes it unique among open world games as well. Also, almost always, going off the beaten path to explore something results in a reward or something you weren't expecting, which is just the best feeling.
All of this is true, but just the fact that the game has puzzles marks it out as different from most open world games. That you actually interact with the world, rather than just moving through it and fighting things. It's the mechanics that are special here rather than the world map itself.
For example: jumping off the top of a waterfall to split a pedestal with a fucking trident. That's the kind of natural interactivity / puzzle that other open world games just don't have. And I do love those other open world games. But the world is a setting rather than a playground.
I actually think the world / map in Zelda is getting too much credit in this thread and elsewhere when the real stars of the show are the mechanics, controls, ruins, enemies, enemy AI, puzzle design, quest design etc.
Saying 'Wow I can actually go to that mountain!' is meaningless - you can do that in any open world game, pretty much. It's what you can DO there that makes it different.
If this game WASN'T open world, but had all the same mechanics, enemy encounters / AI, villages, runes etc, it would easily be the best 3D Zelda of all time. I think that gets lost here.