The new interface (Unity) requires 3D graphics to run correctly. If your graphics card isn't capable of 3D Compositing, then Ubuntu will fall back to the traditional Gnome desktop that you were used to in Ubuntu 10.10.
In some cases your graphics card will be new and powerful enough to run Unity, but you'll still fall back to the classic desktop. The problem then is that Ubuntu ships with open source graphics drivers for legal reasons, and these drivers may not be advanced enough to run Unity. In this case, you should be able to install the official, proprietary graphics drivers supplied by your graphics card manufacturer (Nvidia, in your case). To see if proprietary drivers are available for your system, go to System > Administration > Additional Drivers in the classic interface, and Ubuntu should automatically detect and offer to install any drivers it finds.
If there are no drivers available for you that will let you run Unity, you can install Unity 2D from the Software Center. It provides similar features and a similar interface to the standard Unity interface, and next October, it will be the fallback for users who can't run regular Unity instead of Gnome.