Can't remember them all, but the biggest differences are:
7 new civs--Hittites, Sumerians, Dutch, Portugese, Maya, Inca, Byzantines
9 custom scenarios--from Mesopotamia to WW2 in the Pacific
Contacts trading is moved back to Printing Press.
Maps trading is moved back to Navigation.
Shakespeare's Theatre now also functions as a hospital.
Ironclads now require their own technology, following Steam Power *
Radio is no longer a tech *
Forbidden Palace has been modified. *
5 new wonders--Statue of Zeus, Temple of Artemis, Mausoleum of Mausalos, Knights Templar, and some other one I can't remember right now.
Great Leaders can no longer rush Great Wonders. You now need Science Leaders for those.
2 new specialists--the Policeman, which lowers corruption, and the civil engineer, which adds shields when constructing buildings. CEs are VERY useful in your outer cities which are massively corrupt.
2 new governments--Feudalism, only useful if you want a large army but have small towns; and Fascism, which allows a huge army with no war weariness at the cost of losing some population upon switching.
* actually in a Conquest patch. 1.22 is the latest.