Take “extreme colonization.” In the original game, you could be boxed out of the space race if there were no friendly environs in your immediate area for you to settle. Now you can research technologies that let you colonize planets generally hostile to life - water worlds, radioactive worlds, high gravity worlds, etc. These research paths become so important in the mid-game that your neighbors will pay a high price to acquire these secrets, maybe too high. At this point, trading terraforming techs is a great way to buy all your rivals’ research.
The most significant change is an attempt to give the dealings with AI controlled races more character. The game’s default races now have special abilities they can bring to bear in the battle for cosmic domination. And beyond the expanded diplomatic options like new treaties and a more sophisticated espionage model, you will be able to design your own opponents if you like. Given the generally passive nature of many of the alien powers, the opportunity to create a universe of hostile races is too good to pass up.