For some reason there are no other threads about this game. As a long-time fan of the series, I figured I might as well make one. English isn't my first language, so I'll just go ahead and apologize in advance.
Gameplay video (I recommend watching it.)
For those of you who don't know, Sim City is essentially a city building simulator (perhaps not super-realistic however). You build houses, businesses, schools, police stations, bus stops, raise/lower taxes, provide water and energy for your citizens and so on. There are different ways to do everything, so there's a lot of freedom when it comes to how you want to do stuff.
Unlike its predecessors, Sim City 5 is very multiplayer oriented. You can of course play it all by yourself, but I'd say you'll get the most benefit from working with other players one way or another. Part of this stems from the fact that you have various specialization options.
These are the known specializations so far:
Each specialization has its benefits and drawbacks. For example, Casinos will bring you a lot of tourism and raise tons of money, but the drawback is usually increased crime and poverty. Specializing in Education will unlock new technology which benefits your entire region, but large universities can be very expensive to maintain.
Some of these specializations work incredibly well together, and by cooperating with your neighbours you might be able to focus on different things to increase efficiency.
There can be up to 16 different cities in one region, as you can see here; http://i.imgur.com/4I9OZg0.png. The yellow lines in this picture are highways, while the blue lines are railroads. The big circular icons are construction sites for so-called "Great Works" which is a new feature. I'll quote the Sim City wiki; "Great works are huge undertakings that require Sim workers, lots of materials, and funding. *snip* Working together with other cities in your region will help ease the burden on your city and the finished projects benefit everyone.". You can read more about them, and which Great Works you can build, in the wiki.
There are tons of other new features as well, but there's just too much to post here. I'm hoping some people here on MMO would like to start up a region or something once the game is released
You can read more about the game at http://www.simcity.com/en_US or http://en.wikipedia.org/wiki/SimCity_(2013_video_game)