I have to agree that Melo's athletism is average at best. His skills are also above average, not spectacular. He's a good scorer, and good rebounder, but he's not great. Why would he ever become a great player right?
I think while he may not be the super athlete, or the best skills, he has that charisma. He has leadership, and he's just a natural winner. He wins, simple as that. he just has that quality. He's also got basketball IQ, he's a very smart player. He sees the opponent set up certain defensive plays, and he can just breaks it down with a couple easy moves. He makes the game look so easy. In a way he just outsmarts the other team. That's why he's going to be great.
Larry Bird was never a spectacular athlete either, and while he was a good shooter, what made him better than everybody else was because he knew the game, and because he was a winner, 2 important qualities that melo has. it's the 21st century, games aren't just about skills and athletism anymore, it also about using your head.