This is great news. I've been longing for years for a successor to SimCity 4.
The only thing that has concerned me so far in what I've read or watched is that the engine is designed to be able to handle "tens of thousands" of agents. That sounds a bit low to me and I'm expecting city sizes to be pretty small compared to previous games. I think I read somewhere above in this thread that the 'medium' size from Simcity 4 will be used, but even then I'm pretty sure the populations in the millions were still attainable (correct me if wrong - it's been a while since I've played it).
Even with this concern, I'm thinking if the simulation is that much more satisfying that smaller cities will be a compromise I can live with.
I'm not sure what to think about the always online thing. Being always connected to a global market is an idea that appeals to me but it's very difficult to imagine yet how this would play out.