Things I have noticed thus far:
Hordes work somewhat differently from BI. They are both armies and mobile cities (camps, really), with their own buildings. Camps don't seem to have public order, so no rebellions. There are also some mechanics related to how settled people interact with hordes (Western Romans, for example, can recruit from non-hostile hordes passing through their lands).
When conquering a city you have a few new options: you can raze it to the ground, leaving it an uninhabited ruin, you can occupy, sack, sack and occupy, or liberate. Liberating creates a new faction from that city that starts out allied to you.
Religion is a factor here, as it was in BI, but not Rome 2. It affects relations between factions as well as public order in cities.
Family trees are back, and you can even do political marriages with other factions. You can also assign governors to regions in order to either better exploit or stabilize.
The environment in sieges can be destroyed. Tents are knocked over, fields start burning and buildings can be permanently destroyed.
The factions themselves... there are fewer. Eastern and Western Romes, Sassanids, Franks, Saxons, Visigoths and Ostrogoths, Vandals, Alans, Huns. Plus the three nordic factions from the day one DLC. West Rome is supposed to be extremely hard to play as (game rates its difficulty as "legendary", but I've yet to try it.
It's certainly a step up from Rome 2, but I don't know yet how big of a step that is.