The geography and culture of humans is laid out in detailed maps and essays.
This is the key for me, though I think my answer is contrary to Thotas' answer in at least one sense. I have thought for a long time that the 1st ed. separation of race and class for the non-humans stopped one division short. What was the "elf" class in Basic? It was mechanics for a fighter/magic user and expectations of racial abilities and cultural/archetypical baggage too.
So I'd like to see the cultural part moved out of race. (I'd also like to see the archetypes moved out of classes, but that is more contentious.) However, to do that, you really need to then provide the information on cultures (and archetypes). And mostly that is fluffed, though it wouldn't hurt to support that fluff with some light touch mechanics (e.g. mild special abilities in the woods if you come from a woodland elf culture--the implication being that you can learn that same ability later if you spend enough time with the elves).
Once that is done, however, you can afford to make mechanical changes in the races that really mean something, and balance those out. A dwarven resistance to poison is one example. It is in their blood. I think the main argument against is that it is difficult to come up with reasonably balanced abilities for every race to match. Just what is it that humans have that is intrinsic. Here is where I think making it something, "in the blood" explicitly helps, because then you can have a list of things to pick from. Humans would have more choices, being basically widespread mongrels.
Among other things, this would completely remove the need for any mechanical support for half elves and other such races. Plus, if your human was raised by a particular dwarven culture, it is easy to distinguish between what he got from the culture versus what he got from his own bloodline.
Nor do I think this would be terribly complicated. Instead of "pick race" -- surface easy but with some embedded issues related to class and culture, it becomes "pick race, select racial abilities, pick culture, select cultural abilities" -- fewer embedded issues, and pretty darn obvious what you want from the names of things.
Of course, from a balance perspective, it would also be a lot better if any "cultural abilities" were either idiosyncratic and minor, or more about access to things than the things themselves. That is, assume you can come up with a way in the base mechanics to distinguish between ease of learning bows and sneaking in the woods versus a flat bonus to related skills. Then being in a wood elf culture (or similar) gives you more access to learning those things, but doesn't automatically make you better at them. D&D mechanics to so distinguish have generally been underwhelming thus far.