Some great points gitnby. As stated the arm strength you would like to see is between 80-90mph. But more important is the accuracy of the throw and other attributes. The angle the OF takes to get to the ball and make his throw. The ability to play the ball quickly and at the right angle can cut down on a signicant amount of time that can equate the throwing velocity to much higher then one who doesn't get to the ball as quickly and releases as quickly. So bottom line throwing strength needs to be pretty good but angles, approach, and getting the ball in accuratley plays a large role into throwing runners out or keeping them honest.