Baseball prospects are probably the toughest to gauge when compared to other sports. This is largely due to the fact that in many of the other professional sports, the athletes spend time developing in college before jumping to the pros.
These players come into the league and sometimes are thrown directly into the mix. Sometimes they are given a year or two to develop, but there is no minor league system (aside from hockey). Since they spend three to four years in college, it's much easier to predict which young prospects will pan out.
This could be why some major league clubs have adopted a draft system where they try to target college level players. Not only will the players spend less time developing in the minor leagues (hypothetically speaking), but it is also easier to predict their success. Drafting players out of high school is pretty much a crap shoot.
Teams also tend to put too much stock in guys at the lower rungs of the farm system. These guys are often labeled as "top prospects" after playing one year of professional baseball against guys that may never make it past Double-A ball. The lower levels of development is where you often find to most overrated prospects in the system, because there is no telling how that player will develop in the years to come.
With that being said, here is a short list of the Chicago Cubs' most overrated prospects headed into spring training.