Having played the beta, unlike, well, every poster here so far it seems, let's address some of the issues shall we?
City Size: It's more about quality of the city then the quantity of the sprawl. Sure you can fill up the city limits within 10min if you wanted to, but good luck trying to get a higher population. Larger capacity roads, set up some parks to increase the land value, convert those land wasting single houses into large scale condos. Balance the mix of zones to achieve denser ones, as you also need high tech commercial businesses to support the higher wealth of the citizens now. It's actually quite challenging, as after a bit you start setting up a second city to offset some of the load and start specializing.
Computer power: It might LOOK like a simple graphics engine, but believe me, there's a crap load of stuff going on. Every object is its own object agent, it's not abstracted like in SimCity 4 then represented as an animated texture. If you have a traffic jam somewhere, it's because every citizen is actually there trying to get from their home to workplace, and are actually stuck in traffic. As with the above about the quality of the city, better give some alternate, better routes from the residential to industrial zones. Protesters at your city hall in front of said traffic jam? That's 100 individual citizens being rendered with their likes and dislikes and goals, as well as the 100 cars in the jam. It will tax both your CPU and GPU due to the sheer amount of objects on the screen. Then the buildings around it. And visual effects if a fire broke out. And all the lighting, since this is a city building game, so the environment is dynamic, so you can't bake lighting like in FPS games. Lighting is usually one of the most expensive operations in a game, especially with shadows enabled.
It has chugged sometimes on my computer and it's a quad-core Q6600 2.4ghz, 6gb RAM, GTX 560 Ti 2gb, running at 1920x1200. I've turned shadows to low and disabled anti-aliasing to improve performance. While not exactly a powerhouse compared to some current gen systems, it should give you some perspective on how much it can tax your computer.