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.