I would've rather this had been Sakata, but I can't complain much about Kelly.
In addition to first base duties, Kelly will be the baserunning coach (naturally) and be in charge of outfield defense. Kelly's winning teams in Augusta were very much built around small ball, and station-to-station games.
Reportedly, Sabean was asking various players about the managers in the system this past week in Arizona, and how they liked guys. The indications had been promotions (Kelly to CT, Decker to SJ), so obviously this was underestimated.
My favorite Kelly moment: Quitting the WBC, where he was coach of Panama, as a protest over neither the WBC nor the Panamanian Federation allowing him to work with the entire team in practice at any point before the contest.