Accuracy amidst suitable arm strength is most important. That's what keeps Tannehill around even though he lacks so many things elsewhere, notably long term greatness and therefore a belief he's great.
Ability to vary pace and loft to fit the evolving requirement of the play is paramount. If you have someone like that it's like a golfer who has mastered every shot in the short game. Incredible advantage. Players like Clausen and Hackenberg get touted even though all they own is the tunnel vision line drive fastball.
djphinfan predictably had an excellent list. It's so vital especially in the red zone to have that quarterback who is relaxed throughout the dropback and with every segment quickly reviewed and available. My reference point for quarterback stupidity and limitation is 4-wide near the goal line, let's say between the 5 and 10. With some blockheads the only options are dart into coverage or the worthless fade. But the gem with the relaxed mind and relaxed arm can find the path for the semi-floater and a touchdown that feels like theft.
Escapability to run occasionally is the ultimate bonus. But nifty sliding instincts in the pocket is right up there with varying pace and loft in terms of bailing out plays that otherwise look hopeless.
