Johnny Damon saw a large falloff in his slugging percentage in 2010. The previous two seasons he posted a .461 mark, but that dropped to .401 last season. What happened?
Johnny's best slugging areas are a bit doughnut shaped. The following heat map shows his hot slugging zones for 2008 and 2009 combined:
The doughnut hole moved right into the middle of the strike zone, and the dougnut itself became less dense. Damon could no longer connect on what should be the easiest pitches to hit. Note that the move to Comerica Park didn't make the difference, as Johnny's poor slugging came mostly on the road. The doughnut was in the middle of the plate in both places, however.
I suspect Damon lost some bat speed. That would make it more difficult to catch up to pitches over the middle of the plate. He could, however, reach the lower pitches, as he would have a bit more time to get there.
Through his career, Damon played the role of table setter with some power. As he ages, he'll be more dependent on his selectivity to get on base, and will need to leave the power hitting to others.