Some NBA Draft prospects are simply sure things. From LeBron James being drafted by the Cavaliers, to Dwight Howard going to the Orlando Magic, every now and then a team knows exactly what they are going to get when they send their pick in.
Some NBA Draft prospects seem like sure things right up until they set foot on an NBA court. Kwame Brown is widely known as the biggest bust in NBA history, even though he has become a serviceable player in the last couple of seasons. The Wizards will never have another chance at that pick just as the Trail Blazers will never get another chance to draft Michael Jordan instead of Sam Bowie.
Beyond the sure things and the busts, some players fail for reasons that have nothing to do with on court performances. From the grim, like Len Bias' overdose and death shortly after the 1986 NBA Draft, to the unlucky, like Greg Oden's injury woes, some players' fortunes simply don't allow them to ever really make it to the top.
Some players live up to the hype, some never do. Some players have healthy, long careers, some players burn out in just a season or two due to injuries. It is truly impossible to accurately say whether or not a player will be as good as advertised.
Since there is no exact science to ranking prospects the best we can do is predict.