Overpowered = So effective at a particular role (tanking, melee DD, curing, pet tanking, etc) that no other job or job combination can compete at filling that role for a given group/activity.

OR

Able to effectively fill multiple roles for a given group/activity, enabling the group to add more to roles (more DD, more curing, etc) than the content would otherwise allow.

For the record, I'm all for fixing broken jobs, and find most "overpowered" arguments ridiculous.