I'd have to say the aging, aspirations, and the new zoom features. It looks so much more life-like when you can get up close and personal with your Sims.
What i like about sims 2 is how now we can choice wether we want a baby or not, woohoo, mermorys, the rewards, knowing what they wanna be. I like it sooooo much more then the sims 1.
OMIGAWD, where do I begin? TS2 is a superior game to TS1 in so many ways. The graphics, the little nuances of the gameplay such as some of the expressions and recations that sims make. The aging, having an actual pregnancy instead of "Should we have a baby?" question and the baby, completely dressed and swaddled in blankets, appears in it's bassinette. The wants and fears are a great touch. I love that they have memories, it's very helpful to me when I am playing an older sim who was born in-game to see what happened when they were a child, because I don't always remember why some strange want or fear pops up. For example, I had this one male sim get slapped around by a female sim at a party after he kissed another female. I had totally forgotten that when they were teens they had been "involved." I play about 20 diffrent families in my neighborhood, and I try to play them all equally, so I forget who flirted with who two months ago! So for me, the memories are very important.