For me, there is one compelling difference between Greyhawk and the Forgotten Realms. It is this: Greyhawk has kingdoms. The Forgotten Realms doesn't.
While a strict reading of the whole Realms doesn't support that, when you consider the area that most Forgotten Realms stories occur in, Cormyr is at the south-east corner of that area. It's the only kingdom. Elsewhere? City states - Dalelands, Neverwinter, Waterdeep, Baldur's Gate.
Greyhawk, meanwhile, is a place where nationality can really matter. The map is *filled* with nations, with only a handful of city-states. (Of course, the world is named after one of them!)
This opens it up to a different style of campaign than that of the Forgotten Realms. Of course, that campaign is different to the "explore the dungeon" of classic D&D, and that seen in most of the published adventures.
Cheers!