We periodicly play Civ V games as a group that often last for hours, though we don't always finish the whole game. Want in? Bug someone about starting a game, and people will want to play.Game Page
According to the difficulty stats on the Civilization wiki, the only differences between difficulties above Prince are Barbarian Bonus (player's units get extra attack against barbarians) and Barbarian Target Range (how far away barbarian units start running toward player's units and cities). This does nothing to hurt good players, and can create often unfair problems in the early game.
It is agreed by many Civ V experts that civs such as Babylon, Mayans, Korea, and Poland are "God tier" civs, and that they can win with almost any start and any victory condition. Choosing one of these in multiplayer is somewhat cheating, so to keep it fair, no one can pick these.
It is the Enhanced User Interface by bc1. Although it is a mod, it is installed as a DLC, which allows it to work while playing multiplayer. See the civfanatics thread for information.