There is an excellent article analyzing Mike Napoli at Chone Smith's ALL THE WAY blog.
The article posits that Napoli's mastery of a difficult defensive position and power numbers got him to the show, but that his continued success at the big league level comes down to one thing:
Napoli must increase the Batting Average of Balls In Play (BABIP).
BABIP is a stat that separates how a batter is performing when he is not striking out or walking - and since these are two things which Napoli does plenty of, his BABIP may be the ultimate dealbreaker on his tenure at this level.