I was just thinking a thread like this needed to come back, and lo and behold, here it is!
I find that multi-tasking, the calendar, and the gallery are three features to TS4 that I can't imagine any future Sims iterations existing without, that's how vital they've become to the franchise. And as it was said above, I do love toddlers in this iteration. They so adorable it's almost impossible not to love them. I was a die-hard TS2 fan, and TS4 has really scratched that itch for me in a way TS3 just never could. Don't misunderstand me, I spent a lot of money on TS3 for a reason. World Adventures got me into that series, and I was hooked ever since. I just needed a better way to play rotationally because that's the kind of simmer I am. One last feature I'll mention that I don't think may simmers think about too often is the aspiration system in this game. Always wanted to be able to switch from aspiration to aspiration, and now that I can I do it all the time. Really enhances gameplay for me.
The devs really created some features this time around that are absolutely fantastic. Features I didn't even know I wanted, but I absolutely could not live without. I'll be playing this game for years to come.