In fact, Glaus had much better power at 21 years old than Longoria did, and his plate discipline was a bit more advanced. The only problem with comparing Glaus to Longoria is that Glaus had a total of 227 plate appearances in the minors before being called up to the majors, so the chances of the numbers being skewed due to such a small sample size is relatively high.
With that in mind, I dug around and found a few other third basemen that showed equivalent potential at an early age.
(Please note that I only included each players most extensive stats from AA for the sake of comparison.)
I was amazed to find that Adrian Beltre and Eric Chavez both posted better numbers than Longoria at younger ages. What ever happened to those guys? Those were elite caliber numbers they put up, and now they both struggle to hit over 25 home runs in a single season. And Ryan Zimmerman, while not as impressive at AA, has seen a steady decline in his production every year since his rookie campaign.
And what about Glaus? At 23 years old he hit 47 home runs and posted an OPS of 1.008, followed by 41 home runs the next year. Since then, his career OPS has dive bombed to .858. So, what's the deal here?
There really is no simple answer, but the most basic of answers is injuries and steroids. Each of the above players has spent some time on the DL due to shoulder, back, or other injuries (including a ruptured appendix that Beltre suffered back in 2001 in the Dominican Republic), and at least Glaus' name has been linked to steroids. Of course, injuries and steroids have not been confined to young, talented third basemen, so there could be some other common thread linking the demise of these 4 players, but there aren't many other positions on the field that require quick, high impact dives and long, akward throws as frequently as third base. Other than catcher, it is perhaps the most physically demanding position on the field, and can quickly grind away at a player's back, knees, ankles, and shoulders, all effecting bat speed, blance, torque, and power generation.
Is Evan Longoria destined to become another talented, yet oft injured third baseman? Not necessarily. But, his ability to stay off the DL will likely determine whether he is remembered as just another good third baseman, or a Hall of Fame level player.