It is hard to argue that the quarterback position isn't the most difficult in all of sports to evaluate and project from one level to the next. That especially rings true when projecting from high school to college. Moreover, there is no position on the football field that requires more critical examination from the neck up than the quarterback spot.
For decades, scouts and coaches have been duped by becoming enamored with physical tools only to find out the player lacks savvy, instincts, anticipation and/or mental toughness. These are traits that don't show up in a workout environment or combine setting and that bring to mind guys like Ryan Leaf and Tim Couch. It's not all about arm strength and size.
The quarterback must have a great feel for the game, excellent peripheral vision and an innate sense of the offensive philosophy. Perhaps most of all, quarterbacks must have mental toughness.