I have a few disagreements here but in any case, for your viewing pleasure, here are the top 10 of John's Top 20 for 08 (hit up the link for the rest):

All grades are EXTREMELY PRELIMINARY and subject to change.
1. Joba Chamberlain, RHP, Grade A
2. Jose Tabata, OF, Grade B+
3. Ian Kennedy, RHP, Grade B+
4. Alan Horne, RHP, Grade B
5. Austin Jackson, OF, Grade B-
6. Bradley Suttle, 3B, Grade B-
7. Dellin Betances, RHP, Grade B-
8. Austin Romine, C, Grade B- (hate grading guys who haven't played yet, could be C+)
9. Jesus Montero, C, Grade C+ (borderline B-)
10. Dan McCutchen, RHP, Grade C+

Here are what the grades mean:

Grade A prospects are the elite. They have a good chance of becoming stars or superstars. Almost all Grade A prospects develop into major league regulars, if injuries or other problems don't intervene. Note that is a major "if" in some cases.

Grade B prospects have a good chance to enjoy successful careers. Some will develop into stars, some will not. Most end up spending several years in the majors, at the very least in a marginal role.

Grade C prospects are the most common type. These are guys who have something positive going for them, but who may have a question mark or three, or who are just too far away from the majors to get an accurate feel for. A few Grade C guys, especially at the lower levels, do develop into stars. Many end up as role players or bench guys. Some don't make it at all.

A major point to remember is that grades for pitchers do NOT correspond directly to grades for hitters. Many Grade A pitching prospects fail to develop, often due to injuries. Some Grade C pitching prospects turn out much better than expected.

Also note that there is diversity within each category. I'm a tough grader; Grade C+ is actually good praise coming from me, and some C+ prospects turn out very well indeed.

Finally, keep in mind that all grades are shorthand. You have to read the full comment for my full opinion about a player, the letter grade only tells you so much. A Grade C prospect in rookie ball could end up being very impressive, while a Grade C prospect in Triple-A is likely just a future role player.