As you all likely know, version 40.xx was the world activation release which moved a lot of history gen activities into gameplay.
World activation:
All hist figs in the world now get married have children and do their various jobs, outside your fortress. And all their children do the same. (people are continuously simulated while you play your fortress all over the world)
Kings and other nobel positions of all civs properly shift when old nobels die and people can rebel, assassinate, among other things to shift politics to their agendas.
NPC Sites are built, conquered , and destroyed, and reclaimed while playing.
Armies actually move around the map while playing, and they all do this, not just the armies attacking your fortress.
Bandits actually harass towns people.
Civs need a strategic reason to attack you, gone are the days of invasions every year, now if you get the attention of a hostile civ and they have the resources to attack you and they have an agenda, that makes them want to attack you, they will then attack.
Many many other processes from history gen have been brought into gameplay as-well.
I suggest someone (not me as I do not have the time) create a wiki article about this important mechanic as a lot of people don't understand what this term means let alone the effects it has on gameplay.