I've learned a lot about prospects from reading John Sickels over the years. John gives prospects letter grades the same way American schools do, and he's a pretty tough grader. He's never given an A , but recently described what one would theoretically look like.
A Grade A pitching prospect would have four plus-plus pitches, exceptional command and control, a great body, perfect mechanics, no injury history, outstanding makeup, and a brilliant performance record. Again, I've never seen anyone like that. There is always some flaw somewhere, no matter how minor.
I think I've seen someone who comes pretty close to that: Yu Darvish. Yes, it's probably unfair to throw Darvish in with younger minor league and amateur prospects, but he's young enough at 25, and still yet to throw a Major League pitch.
Against John's criteria, the only question marks for me are whether Darvish's best four pitches grade as plus-plus, or merely plus, and how "perfect" mechanics are defined (Darvish has tinkered with his delivery over the years). Aside from that, everything else is there - the physique, makeup, health record and performance record.
