I thought it would be interesting to post the scale that MLB scouts use to grade out prospects. I found this information in the Baseball America Prospect Handbook and it provides a clear look at the system used by scouts and what the system means.
The Scouting Scale
When grading a player's tools, scouts use a standard 20-80 scale (or, some variation thereof). When you read that a pitcher throws an above-average slider, it can be interpreted as a 60 pitch, or a plus pitch. Plus-plus is 70, or well-above-average, and so on. Scouts don't throw 80s around very freely. Here's what each grade means:
80 - Outstanding
70 - Well-above-average
60 - Above-average
50 - Major league average
40 - Below-average
30 - Well-below-average
20 - Poor
A player's overall future potential is also graded on the 20-80 scale, though some teams use a letter grade. This number is not just the sum of his tools, but rather a profiling system and a scout's ultimate opinion of the player.
70-80 (A): This category is reserved for the elite players in baseball. This player will be a perennial all-star, the best player at his position, one of the top five starters in the game, or a frontline closer. Alex Rodriguez, Ichiro Suzuki, and Johan Santana reside here.
60-69 (B): You'll find all-star-caliber players here: #2 starters on a championship club and first-division players. See John Lackey, Torii Hunter, and Carl Crawford.
55-59 (C+): The majority of first-division starters are found in this range, including quality #2 and #3 starters, frontline set-up men, and second-tier closers.
50-54 (C): Solid-average everyday major leaguers. Most are not first-division regulars. This group also includes #4 and #5 starters.
45-49 (D+): Fringe everyday players, backups, some #5 starters, middle relievers, pinch-hitters, and one-tool players.
40-44 (D): Up-and-down roster fillers, situational relievers, and 25th players.
38-39 (O): Organizational players who provide depth for the minor leagues, but are not considered future major leaguers.
20-37 (NP): Not a prospect.