Thought this worthy of a new topic. The link above will send you to a fairly in depth study of the success and failure rates of top prospects. It's well worth the read. For the lazier among us, I'm pasting the conclusions of the study below.
About 70% of Baseball America top 100 prospects fail.
Position player prospects succeed much more often than pitching prospects.
About 60% of position players ranked in Baseball America’s top 20 succeed in the majors.
About 40% of pitchers ranked in the top 20 succeed in the majors.
About 30% of position players ranked 21-100 succeed in the majors (with the success rate declining over that ranking range from about 36% to about 25%)
About 20% of pitchers ranked 21-100 succeed in the majors (with the success rate declining over that ranking range from about 22% to about 15%)
The success rate of prospects (both position player and pitchers) is nearly flat and relatively undifferentiated for players ranked 41-100, and especially those ranked 61-100.
Corner infield prospects and catchers are the most likely to succeed in the majors, but outfielders, third basemen and shortstops are the most likely to become stars. Second basemen and pitchers are the least likely prospects to succeed in the majors or to become stars.
Prospect success rates have not improved much over time and there is little data to support the contention that prospects are more likely to succeed now than they have in the past.
I don't know that this would change my mind at all when comes to, for example, trading several top prospects for Greinke. But it was an interesting read. It's really really hard to get good starting pitching. Unless you have a top-20 caliber arm, there is an 80% chance that that pitcher will not experience success at the ML level.