I have been typically playing standard speed, Emperor level, random map types.
I just finished two games where I was unchallenged late in the game. It was around turn 330, and I was third in tech behind two AIs. I basically took control of all of the CSs less two, which I chose not to pursue. I had little competition from the AI for the CSs. Neither of my two competitor AIs appeared to be heading towards any type of victory. Just to get the game over with, I built the UN, and let my CS advantage take me to a victory. It happened in both games.
Suggestions:
The AI needs to more aggressively pursue victory. Diplomatic victory is way too easy.
The AI needs to be more aggressive, especially when the enemy is separated by ocean. I basically sat back in one of the games (archipelago type map) with one warrior for ages before I was attacked. By that time, I had built three ARTILLERY, and I easily fended off a pathetic attack by the AI. I even had Elizabeth in the game, and with her ocean going advantages, she never attacked me. In CiIV, Ragnar would have ripped me to pieces early.
The AI needs to pursue CSs mor aggressively. In this way at least a Diplomatic victory would be more difficult. It would also make a cultural victory more viable.
The AI needs more robust and coordinated attacks. Sending in two rifleman against cities with artillery is just silly and doesn't make sense. When attacking a distant continent or land mass, the AI doesn't properly set up an armada of units with defending ships, which would be necessary for an assault across an ocean.
The AI needs to seem more intelligent. The moves it makes often don't make sense. Sending a lone siege unit after a city is simply ridiculous. Sending one or two older units after a city is equally ridiculous.
I can often go to war without any intention of actually fighting. I sit back while another civ does all the fighting, and I get a ridiculously generous peace treaty with gold, gold-per-turn, luxury and strategic resources, and open borders. It doesn't make sense.
If you want to make CiV a better game, fix the AI.