What is significant that is missing? It's not Shenmue good I know. The npcs don't go home and sleep afaik etc...
But I've had some great interactions with multiple groups. Although I guess they did advertise that - animals thrown into the mix. Although it is kinda annoying when the rhino auto targets you.
Animals are like small scripted bits. But you see very little variation on these sorts of things. What you call interactions are really just scripted events that pop-up in selected locations. They added enough, and so you have an illusion of a world.
It's an illusion of interaction.
"World" games versus "linear" games is simple: in linear games one elements leads to the next. In a true world game all elements are connected together. The result is: complexity.
Again, the Far Cry 4 scripted bits are just manually assembled bits. They are always contained in a fixed spot. So they fake the complexity that would define a true "open world".
Complexity, in general, means that every element in a system is connected with the others. More elements = more connections = more complexity.
In FC4 the protagonist isn't the world, because, again, the world is relatively static outside the scripted events. What is non-static is only the player. But this is exactly why it's not a true open world (nor it even wants to be one, FC4 is very obviously a theme park).