I don't care if it is beaten to death or not. This is a great debate, no matter how many times it has been rehashed.
My humble opinion:
'player' = 'career achievement'
'talent' = 'physical ability/skill level'
A great player is one whose achievements are maximized by things under their own control (i.e. skill) as well as things that aren't necessarily under their control (i.e. surrounding teammates, era/style of play, health/longevity).
Wayne Gretzky is greatest player of all time. His skill level combined with great circumstances provide achievements and statistics that are awe-inspiring.
Mario Lemieux is the most talented player of all time. His combination of physical skills (size, speed, hands, vision, leadership) are unmatched by any player in hockey history. However, his circumstances didn't allow his true greatness as a player to ever be fully tested.
Some can say that to a degree, his circumustances were under his control. It is well known that Mario, in his younger days, was anything but a health nut. Smoking. Junk Food. Lack of phyiscal training. None of this could have been good for his physical condition. But things like the playing in the clutch/grab era of the NHL, genetic back problems, and cancer can't really be categorized as things under his control. But in my mind, there is no question that, when healthy, he was the greatest skill and talent to ever play the game.
His career achievements are awesome, but nowhere near what they could have been if he had the same set of circumstances (health and surrounding talent) that Gretzky had. That's the ONLY reason that Mario Lemieux isn't a no-brainer as the greatest player of all time, and Wayne Gretzky is. The fact that most people - fans, players, coaches, etc. - all have the same comment in this debate 'imagine what Mario could have done....' proves, to me, that Mario is the most skilled, talented, player to ever play the game of hockey. Period.