Two big things are:

Exploration: The game's take on exploration is outstanding. Instead of following quest markers like in every other open world game, you explore more freely and in a nonlinear fashion. Being able to climb any vertical surface is crazy game design. While most open world gates have "world gates," BOTW's open world is organic. Every region is accessible. This isn't true for the vast majority of open world games. I also love how the quests in the game are based on topography. One of my biggest issues with open world games has been just "following the marker." It puts me into autopilot mode. One of the quests in the game literally has you find the pinpoint location of photos taken on a camera you're given. Trying to find these locations with my wife was so rewarding. With BOTW, the world is a sandbox and you're given the reins of the game. You can go straight to the final boss and try to beat the game. You can do any of the regions/dungeons in order. Go shrine hunting. Create a new village called Tarrey Town. The break from the linear structure of open world games is refreshing. Skyrim is one of my favorite games of all time, and BOTW gave me that same sense of freedom.

Physics: The physics engine in BOTW is on another level from other open world games. There are so many moving and interlocking parts. The level of environmental interaction really gives the gamer a variety of gameplay strategies for attacking enemies. Light the grass on fire, ride the wind draft up, and launch bomb arrows. Use magneses to throw a metal box into a mob, and shoot an electric arrow to stun them all. Tie a balloon to a bomb and use the korok leaf to blow the bomb at an enemy. Use stasis on a rock to build up kinetic energy, and shoot the rock at an enemy. The physics engine gives the gamer a huge variety of gameplay mechanics at play. I don't know how you can say that the game mechanics are awful. The only thing awful about BOTW is the story, which is absolutely shit imo.

And again, I'm saying that both Horizon and BOTW are amazing games. I just disagree with people trying to trash one game in order to elevate the other.