Gods & Kings is set to include 27 new units, 13 new buildings, 9 new wonders, and 9 new playable civilizations. A new religion mechanic featuring a "faith" resource will allow players to found their own religion which they can grow to a "world-spanning fully customized religion." Diplomacy has been reworked to include espionage, foreign embassies, and new types of city-states (religious and mercantile). City-states will also feature a new quest system and strategic importance. Additionally, the combat system has been re-worked to include smarter AI, an expanded early modern era, and enhanced naval combat featuring Great Admiral and melee naval units.
Three new scenarios will be included in the expansion: "The Fall of Rome", focusing on the decline of the Roman Empire; "Into the Renaissance", focusing on religion in the Middle Ages into the Renaissance era; and "Empire of the Smokey Skies", a Victorian steampunk scenario.
Source: Wikipedia, "Civilization V: Gods & Kings", available under the CC-BY-SA License.