D&D as a system wasn't really all special; there were competing systems back in the days he was at TSR which were every bit as enjoyable and arguably easier to play. But D&D had two big things going for it. First, when the three basic manuals for AD&D were published it had by far the best organized and written materials. The Monster Manual was particularly useful. Second it had the network effect: it was the best system to learn to play because everyone else knew how to play it. You could start a campaign at a drop of a hat -- no need to bring everyone up to speed on yet another set of rules.
Actually, those two things made it remarkably special! It's the overall accessibility and organization that made the system work. While my friends and I tried to start out with Chainmail, we didn't have anyone to show us how so we never really figured it out, and it wasn't very satisfying. But when AD&D came out, we were able to read the books, grasp the concepts, and actually play the game. The game mechanics aren't important, as you can just skip over the awkward rules you don't like (psionics! Bah!) The real magic was the whole of the system didn't hinder our imaginations.
And Troy, if you're reading this, I want my White box set back, please, along with all the other supplements. You've had them for 35 years, now it's my turn.