The problem with stairs might be getting people to negotiate them smoothly. The Sims (up to version 3, anyway) didn't want to deal with collisions on stairs, there's no animation for squeezing past or turning around, so Sims approaching stairs spend a lot of time slowly getting out of one another's way to decide who's going to use the stairs first, and waiting for other Sims to finish so that the next Sim can embark. If you throw a party in a house with stairs, some guests can spend the entire time stuck in the stair queue, and then they go home and complain that the party was no fun.