His going backwards.

Problem with stepping into a championship winning car immediately is that it leaves you no where to move but sideways, which in F1 is always a risk. Mercedes have snapped up a fair few experience engineers and knowing Brawn he will have a ~2 year plan to see the car consistently competitive. Having said that, I don't know how well Hamilton is when it comes to setting the car up, Senna was poor at diagnosing the chassis compared to say Prost but that didn't stop him driving the pants off the car and being accurate to the nearest ~40mm... freak of nature.