tsadkiel
Legend
I think if I wanted to play up the differences, I would point out that they're both kitchen sink settings but the Greyhawk sink has some older things in it - Westerns, the Barsoom books, and quite a bit of influence from historical wargaming, thus the extensive heraldry for all of the nation states.The difference between Faerun and Oerth is small. At heart, they are both kitchen sink settings that don't deviate from the D&D expectations. The difference is scope and small amounts of lore and tone.
It's not as easy as dark and gritty swords and sorcery, though, because both settings have plenty of sword and sorcery influences, and Greyhawk as written isn't really notably darker. (Except for "From the Ashes," but speaking as someone who was running a Greyhawk game at the time, that was a notable shift in tone.)