It's a real shake-up of a system that's normally tied to technology, with each civ independently moving from era to era depending on the pace of their research. Now every civ reaches new ages at exactly the same time, but there’s still a competitive aspect. During each era, civs get points for historic moments, like recruiting unique units or founding a new religion, and at the end of an era these points determine whether the next one is going to be a normal, Golden or Dark Age. These moments can also be viewed in an illustrated timeline of the civ that shows some flavour text and the total number of points they added to the era score.
I'm really looking forward to this.
I wasn't born with enough middle fingers.