From the information available, GALCIV3 is looking pretty much like the previous iteration with nothing that substantial to tell them apart. This is a gut feeling impression and I hope I'm wrong and there's more than updated graphics and tweaks here and there to justify wide consumer interest in a seemingly lackluster premise.
There is a lot of subtle changes. When I play GalCiv 3, I think this is Gal Civ 2 with better graphics. But then I play Gal Civ 2 and go... well why can't I do this... why doesn't this work... and the differences start to become apparent.
1. The adjacency bonuses on the planet tiles are a big difference. Where you build your buildings makes a difference.
2. The updated shipyard designer is not just a prettier version, there is a lot more capability than the last one.
3. There are star ports on the actual gameplay map. In Gal Civ 2, you built ships and structures on a planet. This now has moved to building ships in space and structures on planets. Where different planets sponsor the starport to add to the starports production capabilities.
4. In order to build some ships you need particular resources like Durantium.
5. Ideology tree is new and can give your civilization major bonuses depending, if you are Malicious or Benevolent (or somewhere in the middle)
There are other things, but you can find lists of these things around. I don't know if these changes are just called tweaks. But there are some definite core changes here that you know are not from Gal Civ 2, but they integrate nicely with the game. Also, since Gal Civ 2 was a good game, it would follow that the next iteration should model itself after that game and only add things that worked and remove very little, if anything at all.
I'm not sure what you would consider tweaks here and there. We'll see what new things are being added, but the initial alpha release was a close copy of Gal Civ 2, then they add new things and it's becoming a little more substantial as we continue down the path of development. I can't wait to see what the combat system will bring to the game.