What makes the easy problems easy? For these problems, the task is to explain certain behavioral or cognitive functions: that is, to explain how some causal role is played in the cognitive system, ultimately in the production of behavior. To explain the performance of such a function, one need only specify a mechanism that plays the relevant role. And there is good reason to believe that neural or computational mechanisms can play those roles.
What makes the hard problem hard? Here, the task is not to explain behavioral and cognitive functions: even once one has an explanation of all the relevant functions in the vicinity of consciousness—discrimination, integration, access, report, control—there may still remain a further question: why is the performance of these functions accompanied by experience?
It seems clear that for Chalmers any description in terms of behavior and cognitive function is by definition not addressing the hard problem.
However, see how he contrasts it with the “easy problems” (from Consciousness and its Place in Nature—pdf):
It seems clear that for Chalmers any description in terms of behavior and cognitive function is by definition not addressing the hard problem.
But that is not to say that qualia are irreducibole things, that is to say that mechanical explanations of qualia have not worked to date