To be arguably the greatest hockey player ever, t's almost unfathomable to think that 3 games into his NHL career, Orr had a knee injury, and consequently had TWELVE surgeries on both knees over his career. And keep in mind when Orr played, they didn't have scopes and the technology to make a "clean" procedure. Each of his 12 operations were about going in and tearing the knee up. It really is mind-blowing to think that as great as Orr was, imagine HOW much greater he could have been without his chronic knee problems. Similarly with Mario and his chronic bad back that plagued him throughout much of his career.
The few times I saw Gretzky playing in person I watched him like a hawk. The guy would conserve energy away from the direct action. Gretzky would almost park and hide near the mid ice boards watching everything. His defender would tend to lose focus. Then Gretz would explode and arrive at the right place at the right time and be a couple of steps ahead of developments taking place on the ice. He had an incredible sense of flow and where to be. Orr was completely different. Just give him the puck. He had a commanding presence on the ice and the game revolved around him. Orr could keep the puck away from a team on a power play for almost the whole 2 minutes. Never saw a player like that. You simply couldn't get the puck away from him.