Until 3 years ago, I was exclusively a player, mostly in 3rd Edition D&D with a foray into Star Wars Saga.
Since then I've GM'd Pathfinder and Savage Worlds, and been a player in two separate GURPS campaigns. I've also got my eye on the One Ring, Legends of Anglerre (FATE), Fantasy Craft, and Ars Magica to try GM-ing at some point.
My experience with ENWorld is that almost everyone who regularly posts in the D&D and General Tabletop forums have at least some GM experience under their belt.