I know. I suffer that pain at least...oh... weekly.

Great GMing, IMO, can be as simple as assembling the right cast of players for a game. Preparing the game so that the characters are at the center of the story and the players fulfill their 'wants' in the game. Providing immersion for those who want to achieve it and overall enjoyment as a group.

Sounds simple.

As a GM, great GMing experiences to me is feeling the flow of the game take over and provide the players with a great plot, usually spun right from being inspired by their interaction with each other, NPCs and the setting. To me it is the flow of the game and immersion into NPCs that make pivotal appearances (ask my players about Mad Benedict and Monstrous Llewella from my Dark Amber campaign). Inspiring real life feelings via the game, affecting the players' emotions through the story and its implications.
Those things mean great GMing experiences.