I think it's just the ability to play out specific stories with specific goals, and it might be easier to care for your Sims in the stories series. I don't know, I thought I recall hearing somewhere that it was easier, but I haven't looked into it. Also, the games in the Sims Stories series were optimized for laptops and lower-end computers. There are also less things and options, so once you finish story modes, you're basically just playing on a limited version of the regular S2.