Been watching a lot of roy's early career lately and some documentaries on him. I know several times on these forums, myself included, people have mentioned how Roy is mostly a product of insane reflexes and hand-speed.
However lately I'm not so sure. Roy gets away with a lot of things that would be considered technical mistakes by most, sure, but at the same time RJJ was a master of his craft just like any other elite boxer. Then I think you have to consider what makes a young RJJ that cut above, and for that we have to look at his formative years in boxing.
By most accounts RJ Snr put little RJJ through hell in the gym at a very young age, working him extremely hard and doing stuff like tying one of his arms up in sparring, as well as making him spar older, heavier and much more experienced boxers.
In much of his professional career, RJJ has played his cards very close to his chest. Unlike many fighters who are happy to let cameras roll around in their gym while they work their asses off, attempt to find much footage of Roy doing anything other than relaxed work and you will be hard pressed. He's said himself he's not keen on letting all eyes in on his training camps. This is a smart move as a fighter, he never gives his opponents a chance to see him at his worst or making mistakes and paying for it.
His athletic attributes certainly play a large part, speed is always a dominating attribute to have in boxing, especially combined with his potential to KO guys on the spot. However there are plenty of fighters who have had near RJ's hand speed, and have not been nearly as dominating or successful as he has.