My understanding is that the new SimCity is a "bottom-up" rather than "top-down" simulation - rather than, for example, traffic being a function of road usage based on location of residential areas & how many people a road can support (once traffic exceeds 100% you get "heavy traffic", and lots of cars suddenly appear on the road), the new SimCity instead simulates *each individual car* - they're all there because they belong to a house, and are going to a job. Traffic is therefore completely dynamic - there's no preset "low", "medium" and "heavy" traffic, it is what it is. And it affects everything - heavy traffic stops fire trucks dead, cops cant move through the cities, etc. Power is simulated in a similar way (but I believe they've gotten rid of water pipes, and they're now incorporated into the road).
The new cities are apparently all roughly the same size as the "medium" blocks in SimCity 4 - so you won't be able to build a massive, thriving metropolis in a single map, but it puts more focus on getting a smaller city running well - the limitation forces the player to plan, and possibly rebuild, multiple times to get the perfect city. The limitation is due to the "bottom-up" design requiring a lot of power to run, since everything is being simulated, and trying to make the game accessible to everyone who may not be running high end rigs. I don't think they've ruled out bigger maps in the future once hardware catches up though.
Ultimately I'm looking forward to it, it seems EA has realised Societies was a piece of shit and they're giving Will Wright a bit more freedom to make a more faithful reboot of the franchise - the only thing I despise is the "online required" aspect of the game - it means playing with other players on a single region is a seamless experience, but really, EA, why almost cripple one of the few truly great games you've made in recent years by making it "completely" online?! (you can play a "private" region by yourself, but *still* require an internet connection since so much of the game is server side).