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.  
