Backstories. For all the iterations, the backstories I develop a character with and continue to develop through gameplay are the number one thing. Be it Vlad ruling over all that is supernatural while raising daughters or two rival grandmothers competing to knit the best stuff, once I have a concept of the characters and their identities then my imagination can run wild.
To be fair, I do agree with the criticism, though. With 4 it is all my imagination all the time so my backstories are more important than ever. The game itself doesn't help create the story like I have had happen in the past with other entries. I also do not get so enveloped into playing either, for better or worse. It takes a lot more effort on the players' part because the Sims are more doll like than ever.
I can have fun with 4 and some things it does the best in the series, but I do miss the depth of gameplay we had in the past. 3's personalities especially come to mind.