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VideoGamer visited a Civilization V: Gods & Kings 'behind-closed-doors' presentation during GDC 2012. They posted an article that sums up a couple of details about the new Civ5 expansion pack.
Religion:
• There are 20 different Pantheon beliefs to choose from, such as the God of War or the Goddess of the Hunt.
• Your pantheon choice rolls into your religion, and you'll also get to choose between two more bonus abilities. The first is the Founder belief, which gives you rewards for spreading the religion to other cities, and the second is the Follower belief, which will confer bonuses to any city (and any civilisation) that follows your religion.
• Certain units, like Pictish warriors for the Celts, generate faith when killing other units.
Redesigned City States:
• City states now have a more involved quest system, and can set multiple quests at once.
• Merchant city states have also been added
Espionage
• Spies are now controlled entirely via menus, and don't show up as units on the map.
• There are three tiers of spy; Recruit, Agent and Special Agent.
• The espionage system appears to be more or less like the Civilization V: Beyond the Sword espionage system
Military Changes
• The hit point scale has now been increased to 100 instead of 10.
• Firaxis has slowed down the pace of combat on the whole, believing that combat was unfolding too quickly in vanilla Civilization V.
This and many other details can be found on VideoGamer.com