One of the huge changes between Sims 3 and Sims 2 was the introduction of Story Progression. You probably noticed this first when one of your child Sims' childhood friends grew up, too. Or when one of your adult Sims' friends grew old and died! Or if you switch playable families then come back to your first family only to find there have been pregnancies, marriages, deaths - the usual trials of life.
In Sims 2, time stopped for everyone outside the family you were playing. If you always played the same household (as I did), this created a weird scenario where generations of your Sims would be born, grow up, grow old, and die - all while the neighbors and family friends remained unchanged.
One side effect of this was memorizing and learning to love your NPCs. It was always nice to see a new generation of Sims get purse-whipped by Mrs. Crumplebottom! Another side effect is that you could literally kill off every other Sim in town, leaving your own Sims to repopulate the entire neighborhood. (Several challenges revolved around this scenario, including Last Sim Standing and various Apocalypse challenges.)