He was the 25th draft pick in his class (2009), and was actually drafted by the Angels behind Randal Grichuk (the team had two back to back "compensation" picks). He was a first round pick but just barely, because he was actually the Angels' "second" choice. Normally, a very talented player would be drafted in the "top five," because scouts, coaches and commentators would be all over him. A 20-something draft pick is normally a talented player, but one that teams have doubts about, someone like St. Louis' Kolten Wong.
A talented second baseman named Dustin Pedroia was picked by the Boston Red Sox late in the second round. Teams overlooked him because he was "small" (5'7") for a baseball player. But Trout (over six feet tall, over 200 pounds) had no such disadvantage.
Still, why did so many people overlook his talents to pick him at the end of the first round? Was he a late bloomer who "blossomed" in the minors after being an "indifferent" (high school) player? Were there any obvious "turn-offs" (e.g. health or "signability" issues) that would cause people to "doubt" his talent?