It's hard to put a finger on it.
Countries and personalities with their own agendas. Not just good guys and bad guys.
Lower magic than forgotten realms. But also some almighty weird eldritch stuff, like Vecna.
Detailed but with broad brush strokes.
For me I always felt that forgotten realms was similar in its generic fantasy trope but is trying too hard to be "better than Greyhawk". Like everything Greyhawk has it tries to have one, but better. That doesn't work.
I am just about to start a 5e campaign. I will return to Greyhawk after 6 years in Golarion. Shackled City with a bucketload of extras to make it a sandbox with an adventure path spine. I am setting it around the time of the 3e era gazetteer and am looking forward to the influence of the battle between the Scarlet Brotherhood and the Sea Princes spilling over into Cauldron.
Oh, and yes I would love some 5e greyhawk ( any non massive adventure path 5e really - WOTC doesn't make small adventures I can fit into my Greyhawk)