Even after you start to get a grip on the intricacies of running your city, the new SimCity offers a new level of depth with its online play. No city exists in a vacuum; instead, each city is placed into a region, either with friends or strangers.
Like any real region in the world, what you do in your city affects those around you. This means that regions can work together to fulfill mutually beneficial arrangements (you specialize your government around education and build a college town while I build the hardcore city with nuclear power plants that give us both energy, for instance). Or your towns can compete and / or harm one another.
I might build a gigantic city that churns out coal at a ridiculously high rate, but the pollution Im producing could flow right into my neighbors city, angering his citizens. I could then attempt to offset my neighbors displeasure by giving them cheap coal, or just let them deal with it. My coal-driven city could even affect the larger SimCity world, because now the prices for goods are driven by a world-wide market. If I flood the market with coal the price could drop, or I could build up this city and wait for the price to explode to make a quick bit of cash. Just like real life, the actions of one city affect its neighbors.