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the ability to wait ... the bat hinge and wrists ... the ability to tell a ball from a strike... the lightning-bolt power from line to line...
It is genuinely rare, and right now the main guy you can think of with the same attributes is Edgar Martinez.
You remember Cito Gaston's quote, "You make a mistake to that guy and there is no telling where the ball will wind up."
There's the possibility of Ackley being 100% the hitter that Edgar was, but LH, and playing middle infield. Ackley, in his first few weeks, looks like Edgar at his best.