The DLC significantly expands and strengthens the role of vassals in the game. The Council will demand real influence on our decisions. Each vassal that is subject to us will begin to expect respect and obedience from us, regardless of whether he has any competence. Those who are disregarded may become so offended that they will conspire against the ruler.
The diplomatic system has also been heavily modified to place much greater emphasis on alliances sealed by marriages, coalition formation and the formation of non-aggression pacts. The combat system has also improved. The importance of morale increased and new rules for the functioning of mercenary units appeared.

Makes internal diplomacy more important and volatile. Your large land-owning nobles will demand seats on your council, angering your large landowners can lead to rebellion or attempts on your life so you have to balance ability with appeasement. Can make the game tougher if you like a decentralized empire, but add more to do if you are large landowning noble for a liege. Worth it

A lot of people seemed to take issue with this DLC when it released, but I think it adds a lot to the game; internal politics are severely lacking without it. Definitely disable Defensive Pacts in the game start settings, though.