Measurables, potential, arm talent. All that stuff plus decent numbers in college will get you drafted high. Once in a very great while you hit the jackpot with someone who can really improve in the pros (see Allen, Josh). Often you end up like Mitch.
If he ever has to play significant time for the Bills we are toast IMO. They'd have to go heavy QB runs, he is just way too inconsistent with his passing accuracy and is very non-clutch.