Aside from branching storytelling.. what exactly does the Witcher 3 do?
1. One of the biggest things that Witcher 3 does is that it gives importance to side quests almost as much as it does to the main storyline. The decisions you make in the side quests ripple through the main campaign. Branching storylines is one thing, but having your side quests affect your main story? What other open world game does that?
2. The other huge thing is that unlike most open world games, the game world isn't empty. GTA, RDR, Assassin's Creed, Horizon, all these games have huge worlds that are relatively empty to the amount of content. Witcher 3 almost always has something for you to explore or hunt within reach. Almost everywhere you go, there's hidden loot, bandit encampments, a contract monster, or a side quest nearby...even when you're boating around. Exploration isn't only encouraged, its rewarded.
3. Tons of replay value with entirely different outcomes and an entirely different playstyle.