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From 2007 till 2009, Kelly Johnson was one of the more interesting sabermetrical darlings of the league. He was athletic, played a premium position, walked nearly 10% of the time, struck out under 20% of the time, and flashed some power with the seeming potential for more.
Via Jeff Zimmerman's tool, here are Johnson's swing percentages, compared to league average for his last year in Atlanta:
Against left handed pitching:
Against right handed pitching:
Johnson was a disciplined hitter, pouncing on pitches in the zone, but not being afraid to offer at pitches away (average swing rate). His biggest "take area" was when lefties tried to bust him inside. With this approach, he had two pretty good season and one BABIP driven bad one before trading Turner Field for the relatively hitter friendly conditions in Arizona, and there he took off.
With his new team, Kelly Johnson struck out more (22.1%), but also walked more (11.8%), and his power jumped (26 HRs). Here's how he did it, with his approach in 2010 compared to his approach in 2009.
Against left handed pitchers:
Against right handed pitchers:
Essentially, Kelly Johnson treated opposite handed batters exactly the same, but he changed his approach against same handed pitching, taking more often on the outside corner, and swinging more often on the inside one, as he tried to pull balls for power. The approach worked, as he put up a 149 wRC+ against those lefties he should be struggling with, and a 120 wRC+ against righties.
Since then, however, Johnson has taken this once successful approach to the extreme, with worse results. Here is Johnsons swing pattern from last year, also compared to 2009.
Against left handed pitchers:
Against right handed pitchers:
It's more of the same. The pull tendencies have begun to creep into Johnson's approach against righties, and they're overwhelming his approach against lefties. At the same time, he's taking more pitches on the outside that get called for strikes. My analysis of the situation is that while a little bit of aggression on the inside corner can be a very good thing for a talented hitter like Johnson, he's lost track of the patience, discipline, and opposite field approach that made him so tantalizing back in Atlanta. And without a change in his approach, I find it unlikely that he'll be platoon neutral the way his career stats indicate he may be. I don't want Johnson to become a slap hitter, I just want him to moderate his swing a little bit. When Carlos Pena was at his best, he was trying to go up the middle. The natural talent in his bat did the rest. The same may hold for Kelly Johnson.