I'm not fond of settings that the characters are left to sandbox. What I mean is if you're building a world, you should also have some sort of idea for the players campaign. If it's just aimless and the characters just screw around, that doesn't scream very well organized. It's one thing if that's the intentions of the group.
On the converse side, I hate railroad plots, where the DM forces the group though an extended area whether the group is willing or not. This is also situational, but I'd rather not have long cutscenes of the party walking through a tunnel under a town. I'd rather roleplay that. I don't know if I'm making my points clear in either paragraph, but ah well, i'm tired.
I agree with much of what GenericGuy said earlier. I tend to think a race wouldn't become a populace of any sort through forcible reproduction, and more out of an alliance or actual interest in the other. Otherwise, it wouldn't be a race of its own, period.