Wow -- so many great responses!
What kept me going for a long time were challenges -- ones I designed, and ones other people have designed. But I'm not great at min-maxing and micro-managing. I want my Sims to be happy, and that means making sure they have regular meals and get plenty of sleep, etc.
When TS4 first came out, I was delighted to see that I could play more than one family again. I also discovered the ISBI Challenge -- playing just one Sim in a household. But it's only recently that I've put the two together.
Currently I have 10 families going. I can only control 1 Sim in each of the 10 households, except on all-household days, which my Sims earn by working on skills, getting promotions, making friends, etc. Since Seasons came out, I've been putting in 2-3 hours a day on my game, probably more because I write about them, too.
I can really relate to some of the things that other people have written. Yes, @luthienrising, it is the constant discovery and exploration. I'm like @Holiday in that my Sims haven't been ice-skating yet. Yesterday was the first time they got to make snowpals. And I still haven't tried running a vet clinic like @simgirl1010, but most of my 10 families have a pet. And I agree with @OEII1001 that Seasons has really boosted the fun level. When one of my toddlers dressed in his snowsuit and went outside to play in the snow autonomously, it was a moment of pure joy.
