1. The Sims themselves...
Sims in Sims 4 are more dynamic, extra funny and definitely more fun to play with. The game's main focus is the Sims. There are more social interactions than ever! I can see that the developers paid most attention to sim detail and to the amazing animations. The sim's actions seem so fluid and life-like. Sims will still, however, be sims. They still don't think to remove a dirty dish blocking the refrigerator nor do they think to get out of the way of a fire!
2. The MULITASKING!
This has got to be the most amazing feature ever added to the Sims series! It's so useful, allowing you to satisfy a lot of your sim's needs all at once. Eg. Having breakfast while watching TV and blowing your wife a kiss. It's possible! Sure the other Sims series attempted it, i.e. you could talk while eating in Sims 3 but there are so many more actions that can be performed now. Your sim can now cook or paint while talking to another sim and listening to music!