Harrington was a first round draft pick so that means that athletically he has it all to be a franchise QB.
That said, Mike Martz may not be a great HC but he does know offense, and the passing game in particular.
When he took the job with Detroit he immediately installed Harrington as his unquestioned QB. In thematter of a few weeks in training camp he benched him and never looked back. Then he was traded. Harrington obviously is lacking as a QB. Since its not athletically, it must be between the ears...