I don't know if someone already wrote that - but THE FINANACIAL SYSTEM! :neutral:
I mean, I find myself working very hard to get to the point where my are not swimming in cash. Currently I have a household of 13 sims including YA, teens, children, toddlers and babies (which divide more or less evenly), a butler+maid+babysitter are hired as well - and only at this point they starting to struggle a little like in real life.
Though I did give them "a loan" (=family funds cheat) to buy the house.
Speaking of loans - I wish there was a bank / more intercations / animations that are money related. Not because I'm so capitalist, it's just that I like as much realism in my game. (I know there are mods+CC items for that)