D&D's brilliance comes from the fact that, plain and simply, it has the biggest "brain trust" of any game system on the planet. Like Manchester United, who can draw on football playing talent worldwide, or like the USA, which can siphon off the intelligent and talented multitudes from every corner of the globe, D&D has for decades been the biggest game around, and that means that it can draw on more creativity, talent and innovation than any other system.
The perfect example of this is, of course, the OSR and what is going on nowadays on Google+ (mainly, it has to be said, thanks to this guy more than anybody else). There are hundreds of extremely talented, thoughtful, entertaining and clever gamers pooling their ideas daily on teh internets, and the dead giveaway is that almost (though not entirely) all of it feeds into one game - D&D.
Far from seeing this as a bad thing, I embrace it. Perhaps if I wasn't interested in its own peculiar and idosyncratic take on the fantasy genre I would bemoan D&D's dominance. But I love it, so I don't.