One of the biggest new features that will come with Sims 4 are emotions. Sims are no longer keeping their feelings to themselves, they show you if they are happy, sad or angry. This shows in their faces as well as in the way they walk.
Grant Rodiek (SimGuruGrant) introduced us to the topic. You can get a first impression yourself by watching the second part on the official Sims 4 Gameplay Video which shows part of what we got to see too.