There are few open worlds in gaming that I like as much as the one in Fallout 4. And double so in Survival mode. How the world exists outside of the player, that firefights happen in the distance and patrols can be encountered anywhere and any time. The aftermath is encountered and you wonder what went down here. Scoop the loot and onward.
Two games stand out for me as being perfect for the 'patrol' sort of gameplay I like so much. those are Fallout 4 and the Misery mod for Stalker Call of Pripyat. The sort of open worlds where I can just head out with no objective, no destination and patrol the environs, and find it all compelling.
Yeah, patrol dynamics and exploration are what make an open world for me.