The engine is completely different and holds my biggest gripe:
The maps are much smaller than they used to be and the size of the grid spaces are huge. This, along with the inability to have an early city-building boom makes for small small civilizations. Instead of using cities to provide borders, you are much more relied on cultural borders (even more than civ3). Culture has undergone a rehaul, and is more flexible and complex. This is a good thing. Governments have undergone a complete rehaul, and its much more flexible (you can have a free market, police state, free religion, and slavery!). This makes for great replayability and uniqueness to each civ you create. The graphics and sound, which has always been subpar (but you can't complain because of the gameplay), is OUTSTANDING for this release (the engine is great... zoom into to see the battles, or zoom all the way out and see the whole world!). Each unit talks when you click them, and they do in their native language (I played as the japanese, and all my units were speaking japanese... very nifty!).
I found myself a complete n00b at the game all over again, which left me a bit frustrated. It wasn't for a while that I realized that not only was I never at war, the other civs were being quite peaceful with one another (which would be nice for Sol, but I'm more into the 'conflict brings drama which brings challenges' type gameplay... maybe I just need to increase the difficulty rating from the default).
I was a bit disappointed with my first game, but I think it may have more to do with the fact that I was playing as a n00b instead of a regular civ player like I'm used to. I'm going to start over again and see if that improves my outlook. My biggest dealio last night is was I didn't think "Just one more turn." I was able to turn it off easily (which, as I said, may have more to do with my frustration than the gameplay).
Main point: This game is VERY different and will take time to get used to. Is this a good thing? I'll let you know in a month when I am more of a regular of it.