Gameplay-wise:
Spiral staircase - It would be 4 squares (2X2) and could be placed with any floor level height with no trouble. With the stairs now, it can be a real pain having to twist them all over the place to get them to fit, or line them up right between floors. So a big time saver in build-mode. Going up and down would take the same amount of time for the sim. And honestly, I can't see making a new animation for them being any more difficult than any other animation.
Bunk-bed - Simple. Two sims can sleep on the equivalent of a single bed instead of one sim. (I can see this one being a bit of a problem though programing-wise and animation-wise.) Gameplay-wise it would be really handy though.
