What is your Gamechanger feature?

I'd say the addition of toddlers followed by the Parenthood GP have been the biggest gamechangers. Both have brought a lot more depth to my family play. I just found out that simply interacting with the baby raises the Parenting skill; both my Sims were up to level 2 before their eldest aged to toddler. I enjoy my Sim parents having to deal with the vagaries of childhood phases and teen mood swings. And toddlers are adorable, as well as giving a head start on developing the childhood skills if you get the toddler skills up to level 3 or 5.