If a mediocre QB spends a couple of years playing with a terrific O-line, then they begin to feel more comfortable with holding the ball until the right time, instead of panicing because they are used to getting hit if they hold it for an extra second.
What I am saying is that even what may appear to be a mediocre, or even poor, QB, could be a great QB who just isn't being allowed to become comfortable with holding the ball.
I believe that the NFL goes through alot of QBs that might have been great, but went to a team that placed little value on pass protectors.
So, for me, the most valuable assett that a QB can have is a great offensive line, ahead of any physical talent, or, in most cases, intelligence. (Manning being the one current QB that I would have to say is the exception.)
That's my two-cents.