Exceptional defense? I remember reading he was pretty good, but I don't remember any raving reviews.
I'm not talking Adam Everett level, but more than "pretty good". BA's scouting report after the 2009 season (when he was the Astros #2 prospect) said "He projects to stay at shortstop with a plus arm, great actions, good hands and above-average range to both sides." and they named him Best Infield Arm and Best Defensive Infielder in the system.
Around the same time he was named #2 prospect in the Appy League, and that report said "Mier is a classic shortstop defender who shows great instincts and maturity on the field to go with above-average range and arm strength. His quick feet and impressive technique, combined with average speed, make him a rare high school shortstop who projects to stay at the position as a pro. A need for more consistency in turning double plays and in making strong throws on routine plays were the only critiques of Mier's defensive play, but they're not long-term concerns.
After the 2010 season, they wrote "Mier draws J.J. Hardy comparisons as a solid athlete with smooth actions and sound fundamental skills at shortstop. He's fairly consistent and projects as a possible plus defender with soft hands and a slightly above-average arm. "
After 2011, "A good athlete with fringy speed, Mier has above-average defensive tools with soft hands and an accurate, average throwing arm. He improved at making the routine play in 2011, cutting his errors to 22 after making 34 the previous season. He made better decisions and avoided high-risk plays."
So yeah, "exceptional" was probably too strong, but a SS with above-average range, arm, hands, and instincts is nothing to sniff at. I may be biased, though. I think he could make a bit of a comeback and end up being a decent MLB player.