First, Trout is just 21—he'll turn 22 this August. And thus far in his young career—having played 40 games in his rookie season in 2011, 139 last season despite a brief trip back to the minors, and every game his team has played this year, except one—he seems quite durable.
Second, he already has a spectacular (if short) track record. Not only was he named AL Rookie of the Year last season, but he also had, by consensus among baseball analysts and historians, the best season of any rookie player in the game's history, batting .326 with 30 home runs, 83 RBIs, and a league-leading 129 runs scored. He also led the league in stolen bases (with 49) and was thrown out just five times.
The most hotly debated topic at the end of 2012 was who most deserved the AL's Most Valuable Player award—Trout or the Tigers' Miguel Cabrera. Cabrera won, but it wasn't because he'd had a better season. Anyone who wins baseball's Triple Crown can count on getting the majority of votes from old-guard sportswriters resistant to any new way (meaning, introduced in the last 30 years) of looking at baseball statistics—and Cabrera, sure enough, led the league in batting (.330), home runs (44) and RBIs (139).
Trout, despite playing his home games in a ballpark less favorable for hitters, had just about the same season at-bat as Cabrera, with a combined On-Base Percentage and Slugging Average of .963 to Cabrera's .999. He positively swamped Cabrera in other contributions, stealing 45 more bases, grounding into 21 fewer double plays, and posting much better fielding stats. So far this year, Cabrera, at age 30, is having the best season of his career, and Trout seems to be improving slightly on his last year's numbers, batting .322 at the All-Star break with a .399 OBP and .565 Slg. Also, he seems to have mastered the key defensive position in the outfield, center field, and has started there for many of the 92 games he's played this season.
And third, Trout has versatility. Branch Rickey was the first analyst to define the "Five Tools" for measuring a player's all-around value as the ability to hit with consistency, hit for power, run, field, and throw. It was a rare player, Rickey thought, who was possessed of all five talents. In his 1965 book, The American Diamond, Rickey could find only two players in all of baseball who could do it all: Willie Mays and Mickey Mantle. In the long run, he felt, a team would get the most value from such a multi-skilled player because when he went through a long batting slump or injured his throwing arm, he could still make other contributions to his team.
Trout is not the only "Five Tool" player in baseball today—the Yankees' Robinson Cano and the Mets' David Wright, to name just two players on this year's All-Star squads, can do it all, too. But Trout, crucially, is the only one under the age of 25 and the only player in the game who is ranked at the top by analysts in each offensive and defensive category.