I will sum it up.

Note: major changes only


1: Kill rates run at 1/3 the pace of vanilla. movement is also slower
2: cavalry are much weaker
3: The game is much harder if you play as the romans
4: troops are trained faster; expect to fight more battles per turn
5: Battles tend to be harder
6: Better naval combat(not 3D though...)
7: at least 50 new provinces have been added
8:tons of major reskins
9: new units
10:Many more changes, see the SPQR read me for more details.

Hope that helped