Lots of stuff, I imagine. It's played off as cartoonish, but the Sims universe is packed with awful stuff. Most of the deaths a sim can experience are pretty violent or gruesome, and even if they manage to avoid all of that stuff and peacefully pass away of old age a faceless dude with a scythe shows up at the house and traumatizes the survivors by sucking the soul out of grandma right in front of them.
What's more, they see it happen over and over again, regardless of how they die. So every sim is painfully aware that this is what is waiting for them and their kin at the end. There's no reassurance of "no Johnny, it's going to be okay, that won't happen to you". Johnny, take a good look because this is exactly what you're in for.