My guess is Teamwork, did the WR have high teamwork, because that would make a large difference over time. Plus a lot of people are dead set against training player to play out of their position. so they want anyone at a certain age who will play that role. Then there is the Millen factor as previously stated. there is also the Jerry Jones corollary to the Millen factor in which you get as many bad character guys as possible and hope that they somehow figure out how to be a team.