Here's another prospect from the early 2000s who failed, but his failure was largely not his fault.
Hee Seop Choi was the reason the Cubs let Mark Grace go via free agency. Signed out of Korea at age 20 in 1999, he blew quickly through two levels in 2000 (.298/.383/.557, 34 doubles, 25 home runs in 467 at-bats between High-A and Double-A) and it was thought that by 2002 at the latest, he'd be ready to take over for Grace.
Grace was left out of all this and it's my recollection that management wanted him to mentor Choi, but he wanted no part of it, so he left for Arizona and the Cubs were left without a first baseman in 2001. They signed Matt Stairs to fill in the gap, and later acquired Fred McGriff, and Choi missed half the year in Triple-A with an injury.
Thus not quite ready for 2002, the Cubs retained McGriff, who had a meaningless 30-homer season while the Cubs lost 95 games. Choi had a monster season in Triple-A (.287/.406/.513, 26 homers, 92 walks in 586 PA) and was called up in September... and sat on the bench just pinch-hitting almost literally until right after McGriff reached the 30 HR/100 RBI plateau. He then started most of the rest of the games.
In 2003, he was in his fourth straight year being in Baseball America's Top 100 prospects and won the starting first-base job for Opening Day. He was hitting .244/.389/.496 with seven home runs in 49 games and had become a fan favorite (there were many groups of Korean fans spotted around Wrigley Field with Choi signs) when one of the most sickening injuries I've ever seen happened at Wrigley.
With one out in the top of the fourth against the Yankees on June 7, Choi and Kerry Wood were circling under an infield popup by Jason Giambi. Somehow, neither heard the other and they collided. Somehow, Choi caught the ball, but then fell to the ground. He had suffered a concussion and for a long time didn't move. I was there and I don't think I've ever heard a full house (39,363) so silent. An ambulance actually came onto the field to take Choi to the hospital -- to this day, the only time that's ever happened. Here's a photo of manager Dusty Baker talking to Choi as he's being put into the ambulance:
Interestingly, it was Choi's replacement in the game, Eric Karros, who hit a three-run homer to win it.
Choi returned to the lineup June 30, but he was clearly still suffering the aftereffects of the concussion. In and out of the lineup, he barely played after mid-August and hit just .164/.263/.269 with only one home run in 67 at-bats the rest of the year.
After the season he was traded to the Marlins for Derrek Lee. That worked out pretty well for the Cubs, but not for Choi. He hit reasonably well in 2004 and 2005 with the Marlins and Dodgers (.252/.354/.451, 30 homers in 663 at-bats over the two years), but never played in the major leagues after 2005, and by 2007 he was back playing in Korea. He actually had two pretty good years in the Korean league in 2009 and 2010 and was playing part-time last year at age 36.
But the concussion clearly had long-lasting effects, which we know better now than we did in 2003. It's really a shame, because Choi had excellent strike-zone judgment (.349 career OBP in the majors) and enough power that he might have been a fixture at first base for the Cubs for many years, if not for the events of one sunny June afternoon in 2003.