Yeah, Gygax and the guys did loads of research, especially about rare creatures, old weapons, and old rituals (2nd edition got rid of some of that). They did a lot of work to incorporate the various legends of the world into a single, coherent fantasy world (the couatl being from Mesoamerica, for example).
I wasn't quite satisfied with their treatment on Asian mythology, though (it is a difficult topic to tackle, though) - Oriental Adventures did try to merge Asian mythologies together (mostly Chinese and Japanese), but ended up being like 95% Japanese (for example, only the monk and wu-jen classes were based on China) (Japan was just a small part of Asia, so I am not sure this was justified).
There were various errors as well - shukenja should be shugenja, korobokuru should be korobokkuru, kensai should be kensei, and so on... There was some good stuff in there, though (like the repeating crossbow)
I also never really liked the oriental dragons (from the Fiend Folio); I always thought that gold dragons (and mist dragons) were basically AD&D's answer to oriental dragons, anyway...
But anyway, yeah - couatls are cool

Sounds like a pain to color, but I'll think about it