3D Graphics in The Sims 2
The user will be able to look at a scene in The Sims 2 from any angle. That spoils a lot of tricks we were able to get away with in The Sims.
Wall murals that relied on having a fixed viewpoint won't work in The Sims 2. For example, if you were to shift your viewpoint while looking at this scene on the Topside Mall Skyway from Moon Sims, you would expect to see the window frames moving with respect to the moon base in the background. The only way to create a similar scene in The Sims 2 is to build 3D models of the actual objects -- the moon base, the other equipment outside, and the windows -- and let the user build the while thing in the game.
This same phenomenon will change the way we make lots of other objects that provide extra dimension to the original game. Besides murals depicting outside objects and landscapes, artists have made closets, alcoves, gratings that show subterranean basements and drains; all of these would look really weird in The Sims 2.