I think Voadam brings up a good point and it goes beyond mere kitchensinkism. Most kitchen sink settings take real world analogs as their primary influences; Golarion seems to take Greyhawk as its primary influence, which is a step removed. This reminds me a bit of post-1977 epic fantasy (1977 was the year Sword of Shannara and Lord Foul's Bane were published, setting off a new era of epic fantasy): the primary influence, or at least referential godfather for the vast majority of post-1977 epic fantasy is Tolkien. Later versions of elves were not, by and large, taken from European mythology but from Tolkien, who derived them from European mythology.
I wouldn't say that Greyhawk has the same place in fantasy rpg settings as The Lord of the Rings has in epic fantasy, but it was the first big fantasy setting. Sure, there were contemporaries but none of them were as played and as influential for what came later on. The Forgotten Realms are probably the most popular and played in fantasy RPG setting of all time, but they were published about a decade after Greyhawk and thus have that many years less of influence.
I would also say that the other big kitchen sink D&D setting, the Forgotten Realms, does not seem to draw from Greyhawk nearly as directly as Golarion does. It even seems that the FR is a parallel development to Greyhawk; this would match what I know of its history, that it was created beginning in Ed Greenwood's youth and thus a creative contemporary to Greyhawk, if published later on. Actually, based upon Wikipedia the FR were probably begun a few years earlier, in 1967 compared to 1972 for Greyhawk, but they were published seven years later (1987 to 1980).