It was very much kitchen sink under Gygax's watch - he was unashamedly and openly influenced by Weird Tales magazine, Lovecraft, Marion Zimmer Bradly, Clark Ashton Smith and myths an legends from a myriad of different cultures.
It was Gygax who wrote Expedition to the Barrier Peaks, Dungeonland and Land Beyond the Magic Mirror.
It's probably the inclusiveness that made it so successful compared to other RPGs - pretty much anyone could find an element that they could latch onto and identify with.
Yup, and the reason they make those is they are massively popular.