I'm new to the forum so don't judge me too harshly, but here's my impression of what is compelling about Greyhawk compared to other settings...
Forgotten Realms is the default setting partly because it's the place where things never truly get screwed up beyond repair. Yes Tiamat almost returns and Acerarak almost creates a Death God, but the BBEGs are always foiled, peace returns to the Sword Coast, all thanks to our intrepid heroes (and the lore literally has some strong as hell good characters like Elminster and Drizzt played up). That's a pretty good setting for it's own uses, especially as an introduction for new players.
But Greyhawk isn't that. I won't say it's truly as grimdark as Warhammer Fantasy (where everything is on the verge of total annihilation) or Dark Sun (where everything is already screwed up beyond repair), but it is in this zone where there are good guys, but there are way more bad guys, and even the good guys often make morally dubious decisions to survive. In that way, it feels a lot more real than the "Everything will work out" of FR and the "We are already totally screwed" of Dark Sun.
And in that way, it makes the antagonists (Iuz, Yggwilv, Kyuss, Vecna, and Rary) way more interesting because they linger between "can win, but can also lose." The villains in FR never truly feel threatening because you feel someone's going to beat them. The villains in Dark Sun are threatening but have mostly already won. The villains in Greyhawk haven't yet won, but it feels like they can. I haven't watched Critical Role but that to me is a big reason why Vecna is one of their BBEG.
Anyway that's my pitch. Feels more realistic, feels more threatening, and there's a lot of room for customization of setting and character.