Bought it myself to find out.
Sovereigns. These guys are characters that have their own skills and personalities. Your leader in GalCiv only represents you during diplomacy, and has no individual skills, while here your sovereign is on the map, has personal skills, can cast spells, and can fight.
As such, there's a good bit of creative expression in a sovereign. Yes, I already created my own. He'll probably end up dying horribly.
NPC Monsters are everywhere. There are also non-player factions, which is different - basically they're playing the same game you are, while the monsters are already there and don't build up a civilization. So that's a huge difference.
I've heard that the city-building portion is much deeper in this one, as well, with multiple paths for the city to take shape along. I haven't gotten into them yet.
Overall, it looks like it's a small step up in complexity, while running worse and having worse graphics than GalCiv2.