You make a good point about his fielding percentage, but at the same time, keep in mind that he was only 20, and by all accounts, really should have been at AA.  It's reasonable to assume he was probably focusing a great deal on his offense to try to bridge the gap between High-A (where he had solid success,) and AAA, while his defense wasn't as much of a concern or a priority because he's always shown solid defensive skills.  

Also realize that he's been promoted pretty quickly for a guy who was only 20 years old this year; in 2005, at the young age of 19, he spent time at 3 levels:

192 ABs at Low-A Wisconsin
225 ABs at High-A Inland Empire
23 ABs at AAA Tacoma

I think next year will be a more telling sign on whether his defense is overrated or if he just struggled this year due to the overemphasis of trying to improve his offensive game by two levels at the young age of 20.  He should be more comfortable at AAA next year in terms of his offensive game, which should allow him to concentrate on both aspects of his game more equally.  That should give us a better idea of how good his defense really is.