I chose Greyhawk as both the Forgotten Realms and Eberron steer a bit from the core rules in certain areas.
In terms of magic, FR has the Weave -- which is not part of the core rules. Similarly, Eberron does use the core rules but is not truly a traditional high fantasy world. Indeed, I think that although Eberron does use the 3.5 rules the technomagic aspects of it do not really model the fiction and legends that inspire the game.
Also, Eberron has several races that are not part of the core rules -- or even derivations of standard races. I am not knocking Eberron but I think that it has a flavor that does not translate as universally as the Realms or Greyhawk.
For the Forgotten Realms, it is a more complex setting in some ways than Greyhawk. The world is far more developed, and this might intimidate some GMs who want an established setting but a little room to develop things.