Part of an interview with head Sims 3 producer Lindsay Pearson from PC Game play:

Which Sims 2 elements will find their way to Sims 3?
Some elements were selected from the start because they fit into the core experience of The Sims 3. Fishing for example is one of the expansion features that made it to an important skill in Sims 3.
There's also a Creativity skill, only this time it's based on the guitar instead of the piano.

The Sims is considered an interactive soap. How will you lift this to a higher level in The Sims 3?
The seamless Neighborhood is a huge step forward. The player will be able to go into the Neighborhood and adjust all inhabitants. For example, if you want to model your Neighborhood to The Sopranos or Desperate Housewives, then you can adjust all inhabitants til they are just like those people. And then the game takes over and you become part of that world. So it's actually much more then a TV-series since things happen out of your control.

Will The Sims 3 contain multiple Neighborhood ?
No, there's only 1 Neighborhood , but then it's extremely large - More then 90 houses, each inhabited by a Sim family. But you can save the Neighborhood into different slots so that you can create multiple story lines.
Probably later, we will make tools available so you can create your own Neighborhood , but at release, your stuck with just one Neighborhood .

Are there certain scenarios you can choose?
Well, you can choose from very diverse families which will all create a separate storyline. If you want to play with a family that has 2 children at the start, it will be an entirely different experience then if you choose a single adolescent who's at the start of his career.

Can you tell some more about the Customization options?
With an integrated editor, you can adjust every object, from walls to furniture to fashion and hairstyles. You can create colors and patterns and then use it on all objects. If you want a sofa with zebra motif with purple stripes, then that's perfectly possible. Then you can also use that pattern on your bed or reading lamp.
Also in CAS, you have more control about the looks, clothes, hair, etc. and you can share these creations easily with other players. If you can design cars? Well you can change the color pattern but if you want to add silly things on your car, get Spore.

Do you use Sims 2 technology?
No, we created a completely new engine with new animation and rendering technology because that was the only way to realize the lively Neighborhood and extensive design tools. The engine for Sims 2 was good for what it was meant for, but to implement the Sims 3 features, it was insufficient.
The biggest work was done in the last year. About system requirements, we can't give those yet of course but we do want to make the game available for a very wide audience.

The visual leap between Sims 2 and Sims 3 seems smaller then between Sims 1 and 2. Does this have anything to do with the - probably demanding - Neighborhood element?
Partly yes. However the screenshots give a somewhat distorted image, because when you see the Sims 3 live at work, you immediately notice the high detail in the world and its characters. But yes, the Neighborhood element with its hundreds of inhabitants who all live out a life simultaneously, forced us to make a trade-off between speed and graphical detail. Because of this, we can't make the huge leap forward as with the Sims 2. However a leap from 2D to 3D is always hard to surpass.