I'm finding barbarians to be fierce. Played two games, and in both I spent the first 30 turns being run ragged by barbarian camps on every side. The first time, by the time I stabilised things I was forward settled on either side by Poland and Scythia. Second time was much better!
I really like this, and hope it slows down infrastructure as well, in an average game. Most civilizations struggled with survival, let alone building an organized and stable society. This makes it so that you have to earn your survival, and as a result it may not be as easy to just grow and run through the game unimpeded. Now, some starts will be more isolated, and it will be subsequently easier to develop. But that happened historically, and rather make sense. However, if those civs forget to build a military and focus on that sort of development as well, then you have a chance to overcome them with the military you've refined fighting these barbs.
One cool way, to me at least, to refine this system is to add a mechanic (or better yet, adapt an existing one) that balances the increasing difficulty of managing a larger, more organized, and more advanced society's internal satisfaction with the advantages of growth. Societies that move away from needs to desires often experienced internal strife as a result.
This would create the opportunity for less advanced, smaller civs that are stable to be able to attack a more accomplished civ struggling with internal frustrations on more equal footing.
Corruption, I believe, handled this in other civ games. Amenities are supposed to here, as far as I can tell, but I do not think they are an adequate mechanic.
This would also give another opportunity to refine the differences in the ages (dark, normal, and golden).
