Seamless interconnectivity between areas without loading screens, taking advantage of the engine/hardware for wide open spaces, giving the illusion (or in some cases reality) of very large play space with few boarders and boundaries.
Twilight Princess is an example of "open world" in the sense that you can freely explore large areas, but these areas are designated zones that are funnelled together via chokepoints to hide loading. Hyrule Field isn't really a big field, it's several separate play spaces connected via bridges, tunnels, and paths. When people think "open world" in the modern sense (it's not really modern, but whatever) they think of the former example of larger, load-free play space and not "zones".
To be fair, this won't be the first time 3D Zelda has done this, just the first time it's done it with the detail and geometric diversity required for map predominately based on land. Wind Waker had already done it; massive open play space with load screens hidden during real time play and no zoning. But it got away with doing this by keeping the ocean a mass of relatively empty nothingness, essentially acting as a big simple load screen for the islands. So there was still a sense of zoning, as each island was its own "zone" of activity.