The most noticeable rule change, as opposed to the most controversial rule change of an exhaust blown diffuser, is the height of the nose. From 2011 to 2012, the nose has to be lowered 7.5 centimeters. This has resulted in what some say are the ugliest cars ever to sit on an F1 grid. The "platypus" nose has become the norm on every car except the McLaren (top row, middle column in 2012 picture).
As you can see, McLaren has managed to keep its nose clean by being one step ahead of the game. Its 2011 car (1st car in 2011 picture) had the lowest of all the competitors that season. McLaren attributes this lower nose to their unique suspension geometry that is more compact than the competition. Considering they had a car that was consistently the fastest after Vettel's Kinky Kylie, it would seem that suspension isn't a hindrance to their performance. However, the 2012 Jerez test times aren't as favorable... or they could just be sandbagging.
What else can we deduce from these pictures? Ferrari has taken the criticism of last year's F1 Italia to heart. Both Fernando Alonso and Felipe Massa complained of not being able to keep heat in the front tires which results in a lack of grip and problematic understeer. While we can't see the whole 2012 car (the red one...) in profile here, we can see that the front wing is mounted farther forward relative to the nose of the car than the competition. Either that means Ferrari's nose is very short or they're trying to create a larger moment arm to force the front wheels into the pavement.
Of course, this is all speculation and I could be completely wrong. We'll just have to wait and see what happens at the first race in about a month.