I'm coming to this thread as a Dragonlance fan first, but also a fan of all the D&D settings in one form or another. Greyhawk, the Forgotten Realms, Mystara, and Dragonlance have been categorized as "generic fantasy," which in turn means that products from one setting may compete with products from another setting. Mike Mearls even recently said that he would personally love to produce Greyhawk, but WotC has to make certain that it stands on its own as a setting. Fans from each setting will say how their setting differs, but it isn't like the difference between (example) Greyawk and Dark Sun, or Greyhawk and Spelljammer.
When I think of Greyhawk, I think of old school D&D. This is great for a certain percentage of the crowd, but there's not really much there to get new players involved with the setting. Some of the races of the Player's Handbook, for example, don't seem to have a place yet, such as tieflings and dragonborn.
I do agree with the idea that Greyhawk is much more of a sandbox setting that doesn't have uber-powerful NPCs running around the place. That's to its benefit as well.
I wanted to address a few points from the original poster as well.
4. The classic D&D adventures are associated with Greyhawk. Expedition to the Barrier Peaks, Greyhawk, Temple of Elemental Evil, Greyhawk, Vault of the Drow, Greyhawk, Tomb of Horrors, Greyhawk, Against the Giants, Greyhawk. Even the 3 APs in Dungeon in 3E- Shackled City, Savage Tide, Are of Worms all Greyhawk.
I once thought that Greyhawk could be the "setting of classic dungeons." However, Tales of the Yawning Portal is all about taking these classic adventures and putting them in other worlds, whether published or your own.
5. Classic D&D PHB names, Greyhawk- Bigby, Mordenkainen (AKA Gary's PC), Melf, Tenser, Nystal etc. From the DMG Vecna and Kas.
This is a definite draw for me. I remember getting the City of Greyhawk boxed set so I could learn about all these characters.
Personally, I think Greyhawk needs a few things to make it successful:
1. A new starting point. That could be a slight timeline advance, or it could be a reset a la Dark Sun.
2. It needs to be modernized. I know many old school fans may disagree with me on this. However, to keep Greyhawk alive, you have to have it be enticing to all D&D fans, old school and new. You need those new races, like tieflings and dragonborn. You need the newer classes, like warlocks.
3. WotC should feel free to explore the rest of the continent. There are more areas to explore.
4. Above all, Greyhawk needs to figure out what its identity is and what sets it apart from the Realms.
Anyway, that's my two steel.