One of the things I really love about my Dwimmermount campaign is how readily my five players have all, to varying degrees, contributed to the development of the still-vague game world in which the megadungeon exists. This is particularly in evidence when it comes to the portrayal of the demihuman races.
In our latest session, we discovered a few things about the nature of dwarves, since Vladimir rejoined the party after a long absence. (As an aside, I am gratified that Dwimmermount's dwarves seem to be vaguely Slavic in conception rather than the Germanic or Scottish stereotypes that typically afflict the race). When asked where Vladimir had been while the party was off in Dwimmermount, his player explained that he'd been "making my baby." What he meant, of course, was that Vladimir had been using his time and treasure to carve a son for himself.
As I believe I mentioned once before, my dwarves are free-willed earth elementals who usually don't venture out from their subterranean strongholds. They're thus rather alien in nature, at least by human standards. I decided to sidestep the whole question about what dwarf women look like by establishing outright that there are no female dwarves at all. Dwarves reproduce (slowly) through a long and expensive process of crafting their descendants from living rock. Consequently, most dwarves don't really get the idea of gender or sexual reproduction, leading to all manner of misunderstandings when dealing with non-dwarves.
We also concluded that, because the process of crafting a descendant was so expensive, the newly-born dwarf -- who enters the world a fully functional adult -- is expected to repay his father and his clan, preferably in raw materials like precious metals and jewels. Most dwarven adventurers are in fact on quests either to repay their fathers for the expense of making them or trying to acquire enough valuables to be able to craft their own child. Thus, dwarves aren't really greedy so much as needy; they wish to acquire wealth to ensure the perpetuation of their elemental race and the cultural practices that have grown up around it.
As for elves, it was interesting how many later accretions to the OD&D conception of the race we just accepted without any thought. For example, we never once considered that sleep or charm spells might affect Dordagdonar, because we all knew elves were immune to both. They are in AD&D true, but no such immunity is noted in the three little brown books. Accepting this led Dordagdonar's player to assume that elves are immune to sleep because elves never sleep. They are always awake, which is why some elves, particularly younger ones, quickly grow bored and seek out excitement, even if it is among "ephemerals" like humans. Elven adventurers are thus elves with really short attention spans. We had also previously established that elves have quite the sweet tooth, with sugar acting as a mild narcotic, thus leading to jokes about "Pixie Stix addicts."
And these are all details that evolved through play, springing from tiny germs I'd planted early on in the campaign. I absolutely love this kind of stuff and see it as part of why I love this hobby so much.