The Sims 4, targets for a 2014 PC release, will have "emotion-based gameplay," according to Rachel Franklin, Executive Vice President of the game's development studio, Maxis. She was presenting the game today at an EA Gamescom event in Germany. "From angry to flirty, from sad to happy, you can now explore the gamut of human emotions all within the Sims 4."
Franklin showed off improved home creation tools as well as a Sims character-creator that will allow players to change the bodies and faces of Sims or even change their stride.
The core of the new game appears to be the new way of controlling Sims' emotions. Franklin showed a scene involving two make Sims vying for a woman's affection. She explained that the way she decorated her house, the way she manoeuvred her Sims, affected the Sims' feelings and therefore the outcome of the scenario. Had she, say, gotten one to channel his sadness into working on his art, the woman would have gone with the other guy.
UPDATE: Here's an explanation of the gameplay from EA/Maxis: