Since then, I've been thinking about the various mechanics that D&D has used to implement fantasy. Basic D&D is somewhat unique in using "race as class" (i.e., you're either a dwarf OR a fighter). However, class and level limitations (i.e., a dwarf cannot be a wizard and can only advance until level 8 as a fighter) are common in old school D&D.
In modern D&D (3e, 4e and 5e), dwarf gets bonuses in certain abilities (Strength, Constitution, etc.); in some cases, there are penalties too.
This is somewhat limiting, both in mechanics (by making some race/class combination forbidden or so bad they won't exist in practice) and setting (some setting could have great dwarf wizards, for example, and every people should have its warriors). Of course, the settings might have its build in limitations - dwarven culture frowns upon magic, etc. - but this shouldn't necessarily be built into the mechanics.
In addition, there are people who object to playing creatures that are "dumb" or "weak" on average (although this is circumvented by giving EVERYONE - including humans - bonuses and no penalties... which is just semantics as dwarves are still dumber than humans on average, and occasionally makes humans better or worse than all others).
It occurred to me that having "minimum requirements" for some races solves lots of these (real or perceived) problems, and creates additional positive consequences.
Say, for example, that dwarves need Constitution 9 (assuming you're using 3d6 in order to generate attributes). This means that the most frail dwarves are still healthier than humans, but it DOESN'T say the dwarf is much more likely to be the toughest guy in the party - or that he is much more likely to be dumb or clumsy. It is just that, if he HAS a weakness, it is probably somewhere else. It also DOESN'T say he is limited in his class choices, etc. It does say that most dwarves are at least a bit healthy, which tells you something positive about each creature.
This limits player choice in an interesting way: if your Constitution is lower than 9, you cannot be a dwarf. So, your ability score rolls will suggest (but usually not force) not only class but also race.
It also makes sense. Want to play an ogre? Maybe you need Strength 13 minimum. You could negotiate a frailer specimen with your GM, etc., but usually ogres are strong, although some humans are even stronger.
In addition, this seems like the SIMPLEST mechanic available. It is binary. You do not have to recalculate or set limits to ability score or class. You can get a vague about how dwarf society might function, but you cannot pidgeon-hole dwarf individuals.
Of course, if dwarves get additional powers (darkvision), this has to be balanced somehow, but this is easily done by giving humans a power or two to compensate.