The Hound said:
Is it the ability to create a "world" of your own?
As other posters have noted, I tend to create way more material than the players are likely to see - myths and legends, places "off the map", cultural details, historical figures - out of the sheer joy of adding depth to the world.
For Modern games this means doing tons of research on details with which to pepper the environment. Find a book on a bookshelf? Chances are you can look it up on Amazon to see when it was published and how much it costs. The players may never know it, but it's fun for me and one of the real personal rewards of being the GM.
The Hound said:
To make up campaigns with interesting plots and adventures?
I strive to do this - I don't really know if I succeed, since I'm not big on "plot," but I try to create interesting settings for the characters to explore.
The Hound said:
Being the chief Storyteller or Playwright?
Gawd no!
I'm the guy who moves the scenery and the extras around - the players and their characters are the storytellers.
The Hound said:
Being able to roleplay a lot of characters?
Absolutely! Why be limited to just one character when you can play the entire rest of the world?!?
I like to create deep backgrounds for NPCs (which ties into world-creation) whether or not the players are ever likely to explore the characters. I also like to see what I can do mechanically with the characters, such as figuring out what kind of Craft (visual arts) bonus I can min/max for a 4th level character who is supposed to be a respected photojournalist.
Again a lot of it is work that the players may never see, but I do this for my own enjoyment.
The Hound said:
The sheer ego boost of providing entertainment to several people who are hanging on your every word?
I tend to see my mistakes and goofs more than my successes (such as they are), so no.
Oh, and yeah, the groupies, definitely.