Put simply, Greyhawk is what Gary Gygax considered D&D to be. Whenever he added something to the game, it was essentially Greyhawk he was adding it to. It was influenced by his favorite stories and settings. Greyhawk is, as mentioned, more of a 'Points of Light' game. There are not totally good or totally evil countries, though there are some powerful villains to be found. There are powerful heroes, too...but by and large, they try to avoid direct confrontation. There are heroes, to be sure...and organizations that care more about causes the alignments.
Greyhawk has what any good D&D setting has: ancient and abandoned ruins, fallen socities, evil empires, vast barbarian-filled wastelands, mighty cities, strange magics and mysterious powers. When mention is made that Gods don't normally walk Greyhawk...well, they mean that Generally, the powerful deities don't intervene by mutual pact, except through their agents. Plenty of the many, many demigods do their thang, good or evil. It could be someone as powerful as the demilich himself, Acererak or someone as helpful as the paladin cum demigod, Murlynd (she of the everfeeding spoon).
D&D's lexicon, pre-4e, is littered with Greyhawk references. Saltmarsh (and it's sinister secrets)? The Giants we fight Against? The Tomb of Horrors? The Slavers? The Hidden Shrine of Tamochan? The Apparatus of Kwalish? Bigby? Mordankanien? Otto? Tenser?
Greyhawk has grey layers, of course. The Knight Protectors of the Great Kingdom admit follower of Hextor or Heironeous with equal ease, should they vow to protect the Kingdom and follow the order's tenets. The city of Rel Deven is ruled by a Lich...but he is well loved and by most accounts a fine ruler. There are ancient elven kingdoms, dwarven strongholds, mixed nations and human societies that range from democratic collectives to old school monarchies to evil empires to nomadic barbarian hordes.
Greyhawk, put simply, is everything that D&D is. It can be exactly what you want it to be. No two Greyhawks are exactly the same....and that's the way that Gary intended it to be. It is, in some ways, the exact opposite of Faerun...where Faerun offers tons of detail for the DM to use, Greyhawk offers general ideas. Both are equally interesting approaches, but the reason that it's hard to say what Greyhawk IS owes to the fact that you won't get the same answer from two different DMs.
In my Greyhawk, for example, the God Ralishaz rose from relative obscurity to become the most powerful deity on the Prime, overseeing a host of Neutral Celestials called Judges, who enforced an edict known as the Interdiction, preventing long term summoning of creatures to the Prime. I'd wager no one else's Greyhawk is like that, nor features ancient beings called the Primals who are similar to the Titans of Greek Myth or feature a Triumvarite agreement by Corellion, Heironeous and Pelor. And that's fine, 'cuz that's how Greyhawk rolls. =)
