For as long as I've been a rabid evangelist of limiting the options available
to nonhuman PCs, I've always considered myself pretty permissive when it comes to what kind of nonhuman PCs I allow: as long as it's sentient; it's free-willed; it's capable of functioning in a dungeon, on a ship, or among a (demi)human society and
it exists in the setting, I'm usually game.
I'm happy to accommodate tortles and aranea in Mystara, but not drow, thri-kreen, or halfsies. There are no gnomes, (half)orcs, tieflings, aasimar, or dragonborn on Athas, but I'd love to help you play a genasi, xixchil, gith...
Gets a little fuzzier with metasettings, because I hate the
concept of metasettings but my two favorite AD&D settings are
Spelljammer and
Planescape. Those settings properly... logically... should be
vale tudo but they suffer from thematic collapse if you actually run them that way.
In
Planescape, I strongly encourage players to make Planar characters and I only allow the most vanilla D&D races from the Prime Material Plane: the AD&D PHB races, plus orcs, goblins, hobgoblins, and half-ogres. Other monstrous races are only allowed,
as planars, if they have an iconic planar presence.
In
Spelljammer, I ditch the Great Wheel cosmology and the Radiant Triangle, basically making SJ
not a metasetting. I don't allow
any of the planar races except the githyanki and githzerai; I allow all of the surface-world PHB races (thus excluding dwarves, svirfneblin, and drow); all of the
Complete Spacefarer's Handbook races, plus thri-kreen; and most basic orc, ogre (magi), and goblinoid types. If
Oriental Adventures had ever really included good player races, I'd have included them; the only real contender here is the vanara, and CSFH already has two monkey races.
I prefer whitelisting to blacklisting, but sometimes I have to ban something
specifically when players ignore general guidelines.
Also... this is normally where I brag about what I've done with
Shroompunk, but I haven't ever actually run a
Shroompunk game yet-- I've run
proto-shroompunk games set in the Mushroom Kingdom, using a lot of different systems, but those games were always "humans only, characters are from Earth" and nonhumans were all NPCs. I've got a
stupid number of nonhuman kiths, and I'm using the race-as-class/racial class rules for them... so if someone really wants to play something that's not included, it's going to be an uphill battle. Everything I'm inclined to consider
includable has already been included.
I haven't actually run this campaign, but once I find some free time again, I'd like to try an all-monster campaign where the humans, dwarves, elves, half-elves, halflings, and gnomes are all explicitly the evil villains and banned as PCs.
Chaotic Good Human Ranger who has pledged his twin scimitars in opposition to the tyranny of his evil cousins?
Any time I try to limit any player choice, whatever I'm limiting will be the very first thing the players ask for. Oh, Mr. DM, you want to run a low magic campaign with no core casters? Can I be the only warlock/wizard/sorcerer? (first three character pitches I received after stating I wanted a low magic game).
"Hey, I'm going to run
Street Fighter: the Storytelling Game, retelling the events of
Final Fight and
Street Fighter leading up to
Street Fighter IV and possibly beyond."
"Oh, that sounds really cool! I want to play a character who can't fight and doesn't want to!"
Every. Single. Time.
EDIT: Oh, hey,
major +1 for all the people who treat worldbuilding as part of character building in Session 0.0-0.5. Generally, there's about seven unique lineages in the PHB, and I think a D&D world should have about twice that-- including "standard opposition"-- but there's no reason they have to be the same 7-14 in every single game. Let players whitelist and blacklist things on their own, see where it goes.