How? Thank you for the replies, can you tell me any specifics that are different? Because... similar economic system, similar battles, similar diplomacy, etc..?
Define "similar". Because while on the surface, they might be similar, the differences can make all the difference in the world.
For instance, the economy system has been radically overhauled, IMO. The "adjacency game" turns building placement on its head. The fact that each individual planet is responsible for it's production and research and not just siphoning it off of the entire empire size is a huge change, even if it might not seem like it at the time.
Another for instance. The fact that one can (if one is popular enough) become the head of the UP, means that one can control which measures come up for a vote and which don't. This is a huge change in regards to strategy. Instead of being at the whims of random measures, you have the chance of setting the agenda.
Another change is the getting rid of the "soft cap" of population per world. Presuming it actually sticks, there is noting stopping one from having 100, 200, or even 300 billion population planets. In GC II it took almost every trick under the sun to keep a 24 billion population planet happy.
Then there are added things like strategic resources, trade resources (found on planets - coming in B5) and a whole host of things.
I probably played GC II for well over 4000 hours. And, at first, GC III did in fact seem similar. But it's the what seems to be tiny differences that start to accumulate and make things so much different in the end. I've had to "unlearn" how I played in GC II to get an unstoppable empire and am slowly learning how to do it in GC III.
It might be the difference between Latin and Spanish. Similar at first glance. But radically different in practice.
And all of this isn't getting into things like Ship Roles (which looks like it is going to completely turn combat on its head in GC III - no more only having Uber Kill Huge Ships crush everything without getting scratched) and Carriers.
Plus, you know, 100 opponent maps. 
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To put this all in another way, GC III is like GC II, yes. But it is a deeper experience than GC II. This is intentional according to Stardock. They want it to feel like GC II (and GC I), but have the depth and complexity create a new, if yes similar, experience.
