I've always thought the fact that The Sims was fun was a masterwork of good game design. Especially early in its life when it had fewer of the more fantastical additions - it's a game that simulates normal life down to some pretty mundane details. I mean, the game even started out as an architecture simulator and not an actual game!

Yet, I sunk many hours into the first game as a kid and enjoyed 3 and 4 as an adult (2 was a bit of an in-between for me). Ironically, I think it's the life simulator aspect that makes it fun. It's an escapist sandbox where you can play out "real" life without consequence. Maxis was smart to realize that what was fun was the player's choices, and not so much the game mechanics itself.

Normally (i.e.: the vast majority of the time), games don't simulate pooping.