I readily admit that I ended up GMing D20 games simply because I couldn't find enough players to sustain any other game. When I told my then-potential players that I was going to be tweaking the rules heavily, their reaction was, "But you'll still GM it, right?"
GMo, ergo sum.
I've never had a problem finding a GM, because I am one. In my group there are two other people who like to do "fill in" work in that regard -- giving me a breather now and again, as it were, especially between campaigns. I don't get to play very often and, in the main, I'm not sure if I am all that good of a players. I know I have to hold myself back while in other people's games from thinking "Gee, if I had run that adventure I would have done X, Y, &Z..." Guess that marks me as a bad player...
For our current campaign we are experimenting with co-GMing. We have a world set up and the two others who are interested in some time behind the screen have been given corners of the world; I will do absolutely nothing with those areas other than say, "This area is mountainous" (well, that is for one of them). When the other co-GM gets the bug, we will put the main game on hold, switch to his/her (one of each) corner of the world, play a few sessions with a different group of characters, and then eventually work back to the main story line. Over time the three different story lines might even meet, which could be really interesting.
I have found that a number of people are decent GMs, but that they need a bit of coaxing to get them to try being on the other side of the screen. I encourage a lot of folks to try it. Maybe their first session is not the greatest, but that is no reason to stop. So I give 'em some pointers (
IF they ask for them! I'm not here to force my ideas on them) and suggest they give it more shots. I've trained-up a couple of really fine GMs over the years.
I think as much as anything it is a matter of combined desire and encouragement that makes a good GM. Give folks a shot. Lend them some support. More GMs in this world is very far from a bad thing.