Buchholz' problems at the major league level stemmed from the fact that is fastball, while fast, is deadly straight. For those that remember Calvin Schiraldi, that was his problem as well. His stuff was great but to set it up he needed to throw a real fast fastball but those balls were straight as could be. Major leaguers feast on that stuff.
The other problem, as stated, was in his mind. He was horribly immature and that immaturity lead to confidence problems and problems with accepting coaching when things started to go not so great. Reports are that the kid has really grown up mentally over the last year or so so maybe he comes back to the majors strong.
What I don't like about him is his lack of endurance during the season. He's been needed to be shut down the last two seasons in a row with a dead arm after only about 140-150 innings. He's got a slight frame and if he's showing trouble like this now, his candle could ultimately burn out fast.
For those reasons(straight fastball, early shutdowns, slight frame) I would trade him with a package for Halladay as a Halladay, Lester, Beckett, Wake starting rotation with our bullpen and offense is almost a guarantee of two World Series in a row.