Here's some stuff I've read:
-Ragdoll physics on all the units (that's right: horses, infantry, SHIPS, carts, and so on).
-Building collapsing realistically and hurting units they fall on (Ever wanted to kill some one with there own wonder? Here's your chance
).
-Also buildings and ships are supposed to have realistic damage, like if a cannon ball hits a ships mast the mast will break, or if one hits a house it will put a hole in the wall, and might come out the back side. (What I really want to see is if ships sink differently depending on what hit them where.)
-Instead of small herds of animal there are lots of animals, like herds of hundreds of Bison (like there should be in Old America)