The Kender in 5e thread got me thinking... what races do people think should count as core?
Say you were lead designer for 6e, what would your core races be?
Context Is King. And I'd generally target it at about 3-7 "core" races for any one context (with more added via DMs permission).
In Greyhawk, I'd do hill dwarves and hairfoot halflings and high elves and half-orcs and half-elves and rock gnomes and humans.
In Dark Sun, I'd do mul and half-giants and thri-kreen and cannibal halflings and nomadic elves and oathbound dwarves and humans.
In Planescape, we've got planetouched and bariaur and githzerai and modron outcastes and humans.
In Nentir Vale, you've got the 4e PHB.
In Eberron, warforged and shifters and changelings and khalashtar and....well, Eberron has a lot, but you get the idea.
What I take away from this is that you actually oppose the idea of core races!
I guess from my perspective, core races are races that a player can assume will be allowed at virtually any table. Obviously there will be exceptions, but those will be rare. Pretty much every list on this thread contains dwarves. Presumably if you, as a player, roll up a dwarf you expect to be able to take it pretty much anywhere. If a DM is running a game with no dwarves, I would expect him or her to say so up front and explain the reasoning, because that would definitely be exceptional.
Of course, as you point out, it doesn't have to be this way. I think a PHB that actually defined every race within the context of allowed settings would be cool.