The new tires are 2 inches wider for 2012 than 2011 and they are also heavier due to this. The added weight is a factor especially on a race car where every additional gram is scrutinized. It is also tougher on pit crews who have to deal with a 10% heavier tire and wider/heavier wheel.
The aerodynamics are changed as well. The arches need to be flared to accommodate the new rubber which obviously changes the surface area and drag. The rear wing is also different and much larger but standardized in the effort of controlling costs.
Check out the video below for all the changes and details. Coulthard does a very good job explaining the aerodynamics, wheels, safety, and reasoning for the changes.