Living in the Neighbourhood
Sims 3 will introduce a while new neighbourhood system. In Sims 2 you are pretty much forced to stay at home or visit one of the community lots outside of the neighbourhood. However, in Sims 3 you will be able to walk over to the neighbour and take a cup of coffee. You can jog around and look around, and see and meet a lot of new Sims.
This is a large change from the previous game, and will enable you to co-play with other Sims-families in a completely new way. If you are playing more families yourself, or your real-life friends and family make sims-families in you neighbourhood, you can experience their home and sims in a new way, forming relationships and having fun. Check out the official page for the feature here.