Two points to this I suppose.
I can accept your reasoning for Tieflings. In a game where the world is more dark ages and fearfully superstitious I would agree that character would cause a lot of disruption. However, if that is the reason you give me and then it is not reflected in the game world, that is going to strike me as quite odd. After all, the wizard or dragon sorcerer could just as easily be burnt for practicing dark magics or looking different...
All starting sorcerers regardless of heritage in my game are told that if it comes out that they are sorcerers, they are liable to be burnt at the stake. Some areas give a wink and a nod to such laws and customs depending on whether they know you and how cosmopolitan they are, but in general all sorcerers are assumed to be non-human monsters. Wizards are a different story. Wizardry is 'good' magic, and as long as you follow custom, don't practice witchcraft (mind controlling spells, necromancy, diablerie), and don't abuse your powers - you'll be treated as a 'good wizard' in most areas. There are some areas that don't trust even wizards, particularly in the savage north and remote areas of the western interior.
But I don't think you really do get my reasoning on tieflings. The reason I hate tieflings is their presence basically means that there are no demons in the world, and so along with that no real possibility of that which is alien, and that everything will be treated as basically variant humans in the campaign world. Planescape is a good example; even reified ideas are treated as variant humans. The presence of a tieflings in the world basically means the whole idea of mythology has collapsed in the world and its just become shallow pastiches where you name drop things ('they've got demonic heritage, which means horns and stuff!') that have long had mythic power out of your inability to create anything for yourself. At best, you end up with tieflings as some sort of stand in for a human racial group or simply racism itself, which is itself not that well thought out (people might actually have a darn good reason to discriminate against things with demonic heritage assuming that word demonic has any meaning at all in the setting). At worst, you've got some sort of childish impulse to have a pretense of being grim and gritty because you think it's so darn mature - like saying 'poo poo' and giggling.
So yeah. I dislike tieflings.
Human villagers would be quite hostile to elves and dwarves for being different and strange. It becomes a whole package.
Depends on where you are from. Dwarves are pretty cosmopolitan and get around. Elves are xenophobic, tend to shoot first ask questions later, and often do have mutually antagonistic relations with their neighbors. But there are areas were elves do mix with humans on usually friendly terms.
Second, while I completely get the line about proving ones skill before being allowed to try disruptive characters, it simply rubs me the wrong way. I even agree with it, it just seems like bad phrasing to me
I can't really help that. I've been DMing for 30 years and certain concepts just raise big warning flags for me especially in a player I don't know well. Guy I've never met once to play an evil character or even just a gothic 'black is the new white' character, it sets off big warning bells about player motivation and party cohesion.
My point was a player asking to play a race may be asking for the entire deal, not just a series of personality quirks. That isn't to say gnomes look the same or act the same or even exist in your world, but a gnome is more than a "fey-based trickster with a love of small animals who stands under 3 ft tall". It can be reduced to that point, but reducing by it's nature loses something in the process.
I get that is why you are asking the question you are asking, but if you're going to custom tailor something to fit what they need, is not usually simpler to just let them play a gnome?
No. Not really. Gnomes are considered mythic on my world. There isn't a lot of evidence they ever existed, and most scholars will tell you that they are a myth, or that they are tales invented by the ignorant by encounters with Leprechauns, Forest Dwarves, or Sidhe. Some believe that they were a story invented to illustrate a moral point, which was critical of the Gods, and therefore potentially blasphemous. The truth is not something any living mortal knows, and the gods that do know are keeping it very secret for reasons that are world shaking. A gnome just showing up in the game is well, ridiculous.
In playing a Gnome cleric, to me, that means I should look at the Gnomish pantheon. My characters actual religious practices might change slightly (this particular gnome is actually part of a "cult" and focuses on worshiping the entire pantheon and the idea of community instead of a single diety, and of course "community" could be covered by any number of other deities if I absolutely needed to), but this brings me back to what I was saying above.
Again. The gnomish race is not available. The gnomish pantheon does not exist. Part of that is simply 'I wanted to simplify and gnomes didn't have a lot unique to them to make them worth keeping'. There has never yet been a setting where they had lots of diverse gnomes. Gnomes are consummate single trope races wherever they turn up. Eventually in a completely different campaign I hit upon an idea that made gnomes interesting enough for me that I was willing to even have the word 'gnome' have meaning in the campaign world, although to be honest it's such a rare word most starting PC's would never have even heard it.
And while it's possible to worship an entire pantheon, it's not possible to be a cleric of one. You must pick, again for reasons that are campaign level secrets not known to mortals. But the whole idea is wrong, because the various racial pantheons are intermarried and often inter-worshipped. Indeed, the whole idea that you must worship a particular deity is rather antithetical to the campaign world. Sure, lots of elves will worship Corwin the Holy, but some of them will be perfectly happy worshiping 'human' deities like Lado or Aymara, and conversely humans worship Dianciana and Aerdreth. This is a hugely important point, because most people define personhood as the freedom to worship (or not worship) the deities of their choosing. Creatures that worship a racial deity exclusively are considered little more than fancy puppets or constructs, lacking free will and true personhood. You don't kill them solely out of respect for the deity or for yourself, if you have such respect, and otherwise most people don't consider killing them to be any more murder than squashing a cockroach or killing a puppy (depending on their perspective on life).
So again, I recognize your desire to be in touch with mythic lore that is familiar to you and inspiring to you and to bring in all that overhead of published lore from particular settings, and that's fine, but little or none of it exists where I play. In your homebrew, things are very different and that's great and awesome. And when I'm a player there, I'll want to explore what exists in that space precisely because its not my creation but your invention wholly unknown to me. And that's awesome. But I am by no means required to make my world work like yours.
For me personally, choosing to play a race (and especially a class like cleric which ties so deeply into that) means looking into what all of that will mean.
Great. I've got four page hand outs for people like you giving an overview of each race. But Gnome is not an option. You can play Sidhe, Changling, Pixie, Goblin, Half-Goblin, Hobgoblin, Elf, Human, Half-Elf, Dwarf, Orine, or Idreth. That's it, especially if I don't know you and have reason to admire the maturity of your role-playing.
What I think we're talking past here is reasonable expectations. If I come to a table expecting everything in the PHB to be allowed...
Why the heck would you expect that? That's not reasonable.
My point was that they can be one and the same. A person may want to play a Goliath not only because of their strength bonus and damage reduction ability, but also because of the lore and thematics of the race.
Ok sure, I totally get that. But is Goliath a core 5e race in the PHD? Goliath in 5e is in a supplement not everyone is going to use. So, by your own standards, why would you expect Goliath to be an available option? Moreover, Goliath isn't a race with a very long history in D&D, so if you walked into a 25 year old homebrew, would you expect Goliath to be fitted into the game. Ok, so its great that you admire the lore that the creator of the Goliath concept gave to the race, but why would you expect his homebrew race to get imported into someone else's game just because he got it published?
Someone says, "One of my favorite characters was a Goliath barbarian. I'd like to play that if possible.", then I'll be like, "Cool. Neither Goliath nor barbarian exists as an option in my game, but if you like the idea of playing a massive race then I suggest playing Orine. And you can build a barbarian by taking the much more flexible Fanatic class, customizing it with some wilderness skills, and backgrounding it as being a member of a tribal warrior society. Orine beserkers are well known and feared throughout Korrel, and great Orine tribes roam the steppes of Sartha in large numbers."
Until they have sat down and heard about your world, then they must make decisions based on assumptions.
Why? Why not start with no assumptions until the group has decided what will be played?
If you say you are running a DnD game, and a player is really excited about playing a Tiefling Warlock because they know DnD and see those options in the PHB, then they have no reason to assume that will not be allowed.
I guess that depends on the era you started in. If someone had said they wanted to be a Tiefling Warlock in 1988, once we understood what he was saying, we probably would have showed him the door as a bad apple for even assuming such things would be allowed and then laughed at him when he was gone for such a strange and munchkin request. If someone comes up to me now and says they want to be a Tiefling Warlock, I'll inwardly groan and think, "Oh no. A 4e player. How am I going to cater to someone that actually likes that?"
To be frank, thinking you know anything about what will be played just because you know a group is playing "D&D" suggests to me small and limited experience with D&D. The more groups you play with and the longer you play, the less such an assumption seems reasonable. In some groups, playing anthromorphic Hippos makes perfect sense. I've run campaigns were human wasn't a playable race but kobold was. And the world of variety out there in what people play is enormous. The majority of campaigns are set in very unique homebrew worlds, and many of the published worlds are no less strange. In some campaign you can play a half-giant or an anthromorphic preying mantis, and in others a Tinker Gnome. You shouldn't ever assume that because you played something at one table it's available in another. Playing with a stock setting and default lore is the unusual case.
You see Tieflings as being different then okay, but nothing about them tells me that they have no choice between good and evil.
If they have a choice between good and evil, what do they offer a story that a human can't? The problem with tiefling, among many other things, is that 'demonic' is the only resonate concept that they bring to the game. I don't need a whole race of Hellboys, and I don't need races that have so little to explore in and of itself, and well, 'Hellboy' has been done. I'm not sure where you are going to go with that that won't feel as redundant as a Drizzt Do'Urden clone.