I don't think the build I have is the public build but here are 5 big deals for me personally about Sins Diplomacy;

#1 In single player, I now have some means of influencing relations with different players that I didn't have before.

#2 Three words: Diplomatic Victory Condition

#3 Cruel AI level. Yea, the AI isn't always as intelligent as I'd like but at Cruel, it's pretty bloody challenging.

#4 Envoys. 'nuff said.

#5 Pirate Missions. Now I actually have the option to play the game as an economic power rather than just a military power. The more money you pump into pirates, the nastier their attack.