One of the top three changes made due to community feedback, according to Lead Designer Ed Beach, is that the AI opponent no longer holds grudges that last millennia. He also noted that diehard Civilization 4 players complained there was a lack of depth in the gameplay, which is why religion, diplomacy and combat were added or overhauled. Finally, the combat tech tree doesn't have awkward dead ends anymore. Crossbows can now become gatling gunners, which then become machine gunners.
Producer Dennis Shirk echoed Beach, saying the religion, espionage and combat mechanics were the focus of change. Specific changes affect certain naval units, which now have "melee" abilities that allow them to invade cities after significant bombardment. There's also more flexibility in stacking units, though don't expect the classic "stack of doom" to return. Finally, espionage is handled through one screen with no actual units on the board, streamlining the process of cost and consequence.
The expansion includes 27 new units, 13 new buildings and nine new Wonders, which Firaxis didn't describe further at this event. The expansion is expected to launch in late spring, sometime "before the kids get out of school." Study comes first, kids!