The choice of a QB is determined by which skills are most valued by the front office/ coaching staff. Al Davis put arm strength first because that fit with his ideal down field passing game.
There are a few guys who fill all the blanks in college but most are deficient in one or more areas.
My score card would rank abilities this way:
2 Character (Includes preparation, leadership, focus)
3 Field vision\ post snap recognition;
4 Arm strength\release;
5 intelligence\ pre-snap recognition;
6 toughness (physical, mental)
7 pocket awareness\escapability;
8 running ability;
3 and 5 are the only two that can be readily improved after you're drafted. That requires a craftsman's commitment to improvement ie 2- which is the hardest to project.
A critical quality that is difficult to assess. Is the guy committed enough, smart enough to put in the daily hours beyond learning the playbook? How can you tell?