I've never been heavy on GM's in the group, though it is even more skewed at the moment because of an influx of new players. In the (mainly) long-time group, out of 10 players, 3 have met your criteria of running a full adventure or more. Though with two of them, I wasn't there when they ran it. In the new group, there are currently a total of 14 players possible, though we'll never have them all. Only one has been a GM, and that player is overlap with the first group. Another 2 are getting ready to start, though.
As to what makes a good GM, I find that my biggest problem with many of them is that they didn't get to see me when I had less experience. They mistake some of the things that I do more or less well now as natural talent, when it is really experience. I've had more than one tell me they are uncomfortable running because they "can't measure up"--despite me telling them that it isn't that difficult. Critically, the 2 new players that are seriously preparing to GM are doing it elsewhere, with new players. I encouraged that, as "you'll make mistakes, but keep moving, have fun anyway, laugh about it."