I would think that perhaps more room for the city or a graceful way of adjusting density against the level of curvature might make them more broadly used.1. Curve roads. This is arguably the most important improvement over SimCity 4. If all you want is to build the "most profitable city" or "most efficient city" you probably don't care about this, but for others this is a very important feature.
While this is true for many functions, at the same time it is harder for non-agent statistical models to outright fail, and they can be fixed without as many knock-on effects throughout the simulation.3. Agent based simulation. I know this is controversial, but I think, if implemented well, agent based simulation is much better than statistical model. For example, in SimCity 4, the coverage of a police station is basically by its range, but that's absurd as in real world it's more like to be based on a combination of population and range (also other factors such as basic crime rate). This is hard to simulate with statistical model. For example, in real world, if your streets are jammed with cars, it can be difficult for police to arrive at a crime scene, and reduce the efficiency of your police force. It can be very difficult to simulate such behavior with a statistical model.
I suppose the problem with SimCity is that the implementation has some visible flaws.
This doesn't seem to all the way there, yet. Some of the balance issues may stem from this. I'd imagine a metal or alloys city would be much more rewarding if not having them retarded construction in other cities, since it would follow that skyscrapers and the like need that metal. However, the game seems like it really favors the high-tech and high-education path which is area efficient and gets a lot of mitigation measures, and doesn't value the necessity of the other kinds.1. The global market. The price of some "basic materials" is supposed to be fluctuating based on supply and demand. I haven't observed such fluctuation yet, but if implemented well, this could be a very interesting feature.
Actually, I think that's something they did say you should do. A big portion of that process, and the export/import market is also handled by agent-based units moving in and out of cities, and is subject again to the twitchiness of the agent system.3. Inter-region simulation problem. In theory, it's possible to provide certain services to other cities in the same region, but for what I've seen, it can't really replace "the real thing." It seems that the designers of the game do not want you to create some "garbage city" which specialize in collecting everyone else's garbage, and that's disappointing.
I'm saddened that the worthwhile elements of the game and other topics of debate have been so overshadowed by the always-online component. There are other philosophical/gameplay debates about the game's decisions, such as making so much of the economic system mayor-controlled. How many city mayors run a command economy over their metal and processor exports?