"The first change emphasized by EA is the effort to create towns and true-to-life neighborhoods, where each household functions on its own. Specifically, your neighbors will be up to the same daily routines as you. They'll also get new jobs, get into relationships, etc. Unlike before, in The Sims 3 you can actually see and keep track of each character. In The Sims 2 avatars (be they your neighbor's or your own) would disappear when you're not looking. This time, you can follow them as they go about their day-to-day tasks. If you so desire, you can follow your neighbors' activities - as they start their cars, go off to work and such. For socializing and interaction, EA plans to introduce places like parks, movie theaters, bistros, bookstores, clothing stores etc.
You may have noticed that The Sims 3 doesn't appear on par with most modern-day video games when it comes to graphics. This is because the developers are looking to appeal to the huge Sims fan base and are doing their best to create a simulation of a living and breathing community, in which each citizen remains active as you play, with individual homes, careers, personalities and so on. This, in itself is enough of a burden for PC hardware. Adding more detailed visuals would unnecessarily increase the system requirements for the game."