Unfortunately, the main thing people seem to bring up when they talk about Greyhawk's strengths as a setting are a)it was created by Gary Gygax, and b)moral ambiguity. The first is a moot issue, and the second really doesn't translate reliably without a dedicated DM. Adventures take place on a micro scale, not a macro. I haven't got a real solution for that, either. I like both settings, but the Forgotten Realms are more mainstream and support a wider variety of adventure, much like Golarion.
I would like a tighter focus on smaller areas (ie Vale of Nentir), and if they do a new campaign setting, lose the "sea in the center" design that I feel like I've been seeing more of recently.
...one of the primary reasons I enjoy running/playing Greyhawk while using a non-D&D rules system is because I feel it can be difficult to fit some of what Greyhawk tries to do (and imo does well) into the D&D alignment system; games which use something a bit more nuanced (imo) work better. While you can graft a system like that into D&D, it always feels less whole to me than if I just use a game which has things which can be tagged as character traits.
The micro scale is also something I quite enjoy; though that's also a reason I tend to use a different game system for Greyhawk. In spite of the fact that Greyhawk is one of the original settings, it seems to me that modern D&D has, over the years, evolved into something which has a different mindset and holds different design ideals than the mindset and ideals about gameplay which Greyhawk was designed around.
One of the things I like about the micro scale (if I may mention that again,) is that Greyhawk -while having some similarities to Forgotten Realms as far as being something of a generic fantasy kitchen sink designed for D&D, manages to have a somewhat distinct feel. At the end of the day, your character can be a hero by simply picking up a sword and helping to defend some local girls from being kidnapped or by helping a farmer down the road guard his livestock against some goblins; the world gives just enough of a blank map to allow you and your campaign world to grow into what you want it to be while having just enough detail to also give some ideas about what
might be in those blank spots. While I do also enjoy the
Realms, I feel the Realms often comes across somewhat authoritatively when it comes to deciding what's around that next corner; in contrast, I feel Greyhawk inspires rather than dictates.
In all honestly, there is not; nor has there ever been any sort of rule saying that what's in the books needs to be what's in the world; a GM of any world can change it to suit the needs of the group, and I would expect a GM to make changes if his players enjoy a different path. D&D and rpgs in general are games designed around creating your own story. Still, I feel (personally; in my opinion) that Greyhawk more naturally fosters the idea of growing the campaign into what you want it to be; making the game your own. I enjoy
Realms lore, and it has inspired many ideas for me, but it often feels so full that it's hard to know where to fit in as a player character. To some extent, I do agree that Greyhawk focuses on the micro, but I like that; it's nice sometimes to have a more personal story about the adventure right down the road rather than feeling beholden to rush off across the planes and cosmos to rub elbows with the famous heroes of yesteryear.