This doesn't mean plopable. This actually means you could, for example, upgrade an elementary school to add something such as a gym. Thus improving health. This actually has a lot of potential to add a great deal of depth to the game.
You could technically do this in SC4. All this means is that each city in the region works off each other. Ergo, you could have one city dedicated to residential and another commercial. This further expands the scope of the game.
You can play solo If you want. I'm assuming this feature is there to simply compete with Cities XL.
You're twisting the meaning of this. It just means that sims within the city are more important and less like pawns.
Also, from what I've read, the team behind the sim city reboot consists of plenty SC2k and SC4 veterans.