What makes snow on the wings during take off so dangerous?
it is not obvious if it could alter the wing shape for this to be a problem

It may not be obvious but it is so.

Plain old snow will just blow off as soon as the plane picks up speed, but if the wing was above freezing when the snow first fell on it, the snow will probably conceal a layer of rough ice, which will not blow away. 

Anything adhering to the wing surface will change the lift force. According to this article even a thin coating of ice can disrupt an airfoil's boundary layer airflow, causing a premature aerodynamic stall and dramatically increased functional drag.