There is a difference between aggressive and undisciplined.
An aggressive hitter knows what pitch he's looking for, and isn't afraid to pull the trigger when he gets it.
An undisciplined hitter reacts to pitches and swings at balls out of his hitting zone (less than two strikes) or out of the strike zone.
Herrera is a perfect example, when he was being disciplined, he not only walked more, he hit better because he wasn't chasing bad pitches.
And it's not just strikeouts, Franco is too aggressive and rolls the ball over consistently to short and 3B because he doesn't keep his hands back and wait on pitches.
Some of this is the ability to see the spin out of the pitcher's hand, a lot of this is homework, if a guy has a slider that breaks down sharply, then you don't swing at it when it comes in knee high because if you watched film, you know it's going to end up at your ankles - force him to elevate the breaking ball or take the walk.
I think Williams is going to flop, because he's become less disciplined, whereas Alfaro is learning how to be a disciplined hitter.
And it's why Lehigh is so important, it's where hitters show they can lay off junk outside the strike zone and force pitchers to give them pitches they can handle.
It's not about walks, some guys will draw a lot of walks (especially, if like Herrera, they can foul off strikes they can't handle), other guys use plate discipline to get themselves into favorable counts and force the pitcher to throw something they can hit over the plate.