Bobby Orr came into the NHL during the pre Entry Draft era when sponsorship was the norm. Denis Potvin came into the NHL when junior players had to go thru the entry draft( or whatever it was called at that time - changed a few times) so he wound up playing junior two years longer.Potvin played a few games of major junior at 14.
http://www.hockeydb.com/ihdb/stats/pdisplay.php?pid=4355
In Potvin's case the last two years of junior created anticipation. - Which NHL team would have the good fortune to select him #1? Since the time 18 year olds have become eligible for the entry draft
the hype factor has slowly crept in as there are very few that could play at 18 on any NHL roster like Orr or Potvin could.
Defensemen today coming into the NHL at 18. Doughty had excellent coaching thru junior. Did not see Weber as a junior so will not comment BUT have seen some brutal coaching in Quebec impact on careers - Sauve and Despres come to mind although both seem to have bounced back.Luke Schenn has lagged a bit in his progress.
Defensemen arriving in the NHL at 18 or early. Phaneuf looked better coming in than he does know. Bad coaching combined with a lack of hockey smarts.
Physical maturity is a major factor. Defensemen require greater physicality and are subject to more hitting. Forwards can get by without the same level of physical play.
You are correct about the present day trio but the issue, especially for Ovechkin and Malkin is their play without the puck.
Also of the three Crosby is ahead significantly from the standpoint of the ability to integrate a team - ability to accept/perform changing roles based on teammates and situations.