Too many of these teams draft upon development and potential. They draft guys who will need at least 2 years of development before they are able to play at a level demanded in the NBA. And even the players who are successful often have a glaring flaw in their game. I can look at Carmelo and its his unwillingness to play defense or even LeBron doesn't have the finesse that the great players of yesteryear had. There are a lot of nuances that need to be developed that many of the better players just don't develop in today's game.
I look at Evan Turner as a perfect example of both sides of the argument. He was a late 1st round pick after his Soph year. He made great strides in his 2nd year at OSU but he was still a very raw product. Even in his great Jr. season he needed some work. He improved a great deal, improved to the 2nd pick in the draft, but his ball handling left a little to be desired. He wasn't a finished product, but had shown a great deal of improvement from his Soph season to his Jr season. IMO he made the right decision to leave after his Jr. season but there were still some aspects of his game that could have used some full time tutelage.