The problem with the McLarens is that they historically since Senna left them, is they have never hade a great rear end, Coulthard said it of his time with McLaren (even with Newey) and others have said so as well; Kimi, Montoya and Alonso from their previous teams.
I think that Button has tried to develop the rear end more, but still has to do some work with the mechanichal and aero guys by getting them under the same impression of what is needed. It is the problem with Hamilton that i feel that he has never experinced another teams car to compair the McLarens with. If i were Ron Dennis back when Montoya quit in 2006, once the GP2 season was over, id have tried to have got Hamilton a seat with a smaller team for at least a couple races.
Good cars have a great front and rear end, instantly transfeable in feel and touch. This is where i think Ferarri and Red Bull have the edge over McLaren, their front and rear end solutions work in harmony with each other. McLarens front end battles a little more with the rear end, the front wants to turn, the rear wants to do a little more the other. Hence the McLaren has entry understeer and exit oversteer to my eye from the 2010 cars.
I have a feeling that McLaren will be working on the mechanichal setup of the car and its behaviour this year. Getting it more direct and better under traction as braking has always been a McLaren strength.