In D&D, the assumption is that it's a human world out there. The more powerful demihumans had level limits imposed on them in older versions of D&D, and the benefit of being human was unlimited advancement in your chosen class. A friend of mine recently pointed out that once the level limits were gone, non-human races have no story-basis for not ruling the world (e.g. third and fourth edition). With their lengthy lifespan, how does every Elven wizard not become a world-shaping 20th level archmage? Now, reclusiveness and rarity might be one explaination, but I'd like to focus on the mechanics of it for now. I don't want to claim that level limits are a good way to marry the rarity of non-human races with the rules either.
In the basic D&D game, humans were the default for most class choices. If you played a fighter, thief, cleric, or magic user (or mystic or druid etc) you were human. Elves, dwarves, and halfings were classes and that represented their fixed role in the world. AD&D made race its own thing, but imposed the level limits alongside racial restrictions on classes, so that dwarves, gnomes, and halflings couldn't be magic users (though gnomes could be illusionists). Humans were still the assumed default, but had no mechanical bonus other than the ability for unlimited advancement (hardly an issue when you're below 4th level) and the ability to be classes like the monk or paladin.
In third and fourth edition, humans had more flexibility in their chosen class in other ways (favored classes, bonus feats, or flexible ability scores). The bonus feat is nice, but it means humans aren't the best at any class, just darned good at any of them. Since I've got a mixed opinion of races granting ability score bonuses while class mechanics are heavily ability-score based, I wonder if there's another way to encourage more humans to be played though. One way I'm considering is by the rolling mechanic.
Often players roll up characters in isolation, which can be fine. But when someone has a specific character in mind, they can be a bit irritated if they don't roll well enough to play it (Paladins and specialist wizards were particularly hard to play in AD&D because of stat requirements). Some people are strict about rolling stats in order, but maybe that's where being human could come into play.
In a campaign where you roll for stats, maybe the back-up point buy option is human. Or maybe you roll your 3d6 (or 4d6 drop the lowest) in order, and that's what you play. But if you opt to play a human, you can swap any of those stats around in order to play the class you want. This way a lucky roll is still lucky. So if you were hoping to play an elven mage and rolled a 17 strength and 12 intelligence, you can either switch to play whatever type of warrior you wanted or else play a human and rearrange those stats so you can put that 17 into intelligence. Particularly if you keep the idea of racial minimum stats, playing a non-human has a strong element of luck.
Story-wise, this mechanic would support the view that humans are more flexible as well. Though hard work and dedication, humans would be able to excel at any class, rather than being stratified in a genetic cast system. Many humans could become wizards or warriors with the right training. Elves or dwarves, on the other hand have a fixed lot in life. You could even go so far as to change the racial bonus to rolling better stat-wise. So elves might roll 5d6 (drop the lowest two) for dexterity, intelligence, and charisma. Or perhaps if you agree that you're rolling up an Elf, you roll one extra die that you can swap in for one of the dexterity, intelligence, or charisma dice. Or get an automatic 6 in one of those stats.
If you wanted to expand this type of flexibility, instead of racial bonuses, each race would have one swap stat. Thus, elves could swap any one stat to dexterity (for example) as long as that increases their dexterity score. Dwarves could do it for constitution, etc. Maybe you just want to keep humans as a back-up default point-buy in case someone is unlucky with their rolled stats, but at least you could play the class you were interested in playing. This doesn't say much about class rarity though.
Clearly I haven't thought these character creation methods through completely, and there's a lot of ways the same idea could be implemented. It also crucially hinges on the rolling method, so it's not an all purpose solution for every game. At any rate, its worth considering a number of ways to make humans the setting default that are still unexplored.