Korgoth said:
In D&D, the brilliance of a game session can go beyond the potential of the written rules to express the brilliance of the individuals who are playing. As a game of the imagination, it can go far beyond the letter of the written rule. This open-ended nature allows for imagination, creativity, vision, knowledge and even common sense to supplement the rules and create an infinitely more complex play experience.
The "human element" is what makes D&D great.
Yes, the problem is that, while a great GM can make for a great game, there's really not that many great GMs. In a sense, most people who play D&D (or even RPGs in general) are self-selected, to the extent that we all either are, have or have had a great GM in our play group. I would expect that posters on ENWorld are even more self-selected, with a large majority being comfortable GMing.
That's not the norm. Most people who play D&D do just that; they don't have the time, ability, skill or inclination to GM, and have been lucky enough to attach themselves to someone who can provide them with the play experience they want.
If a group of friends want to play a board game, a collectible card game, an online game, a video game or what not, they go out and purchase the game, might spend some time reading the rules, and then all sit down and play.
In D&D (and most RPGs), the most important part of the play experience can't be purchased in a box set. If no one in your group of friends has the time, ability and skill to be at least a marginally competent GM, you simply can't play, so you move on to some other recreational activity.
The overwhelming majority of the nature of the play experience is based on the GM. It's possible to have a lousy game even with an excellent GM, but very difficult to have an excellent game with a lousy GM.
I could easily be persuaded that removing the GM bottleneck is a necessity if RPGs ever want to be more than a dying niche market.