Some criticisms I have observed over the past few games:
1) City states, particularly maritime/culture city states, are an extremely poorly executed good idea.
1a) Money goes in, food comes out. Greece as an offshoot is overpowered - you are able to maintain a tremendous amount of allies on a very modest income. I just won a one city victory stupendously easily by allying 6 maritime CS for endless amounts of food which gave me endless amounts of specialists which resulted in an extremely easy Culture victory. Not that the other AIs in the game even cared that I was producing 400 culture per turn out of one city, they all seemed pretty okay to just be friends.
1b) Really, Firaxis, that's the extent of my interaction with city states? I occasionally pay them off for set bonuses? Wow you really ran with that idea, eh.
2) Fish tiles are overpowered. Food, gold and production in decent quantity all in one tile.
3) The AI is downright terrible at all things naval. Being on a different continent/island basically ensures your complete safety from any slightly intelligible attack from the AI.