I really didn't like the concept of city states. I much prefer the idea that everyone starts off equally, and if given the chance, could develop into a great civilization. Civ4 had city states if you just fill the map with lots of civs, some would necessarily stay small and you could deal with them how you wanted to, albeit without the gamey bonuses. I didn't like having two tech trees. A civilization knows what it knows, and everything you know is dependent on everything else. You can't have Rock n' Roll without electricity, you can't have organized religion without a writing system, etc. It makes sense to have a winding, diverse path, but splitting the tree in two pretends that culture can advance without science, and vice versa. With both of these contrivances in place, I can't see Civ6 surpassing Civ4.