Interview with Civ IV Producer
IGN has posted an interview with Civilization IV's producer Barry Caudill, who provides more detail on the what's changed since the last Civilization title
3D engine
still covers the 4000 BC to 2050 AD timeframe
revamped movement and combat mechanics
animated, dynamic landscapes (moving cows, flowing rivers)
18 civilizations
better AI
additional resources for trade
new "religion" system for theocratic governments
addition of the touchy (but historically accurate) topics of slavery and emancipation
elimination of terrorism options (poisoning a city's water supply has become a taboo topic since the last Civ game)
the most moddable version of Civilization ever
Visit the folks over at CivFanatics.com for great commentary and analysis on the IGN interview linked below.