Race Design in TTRPGs
Some high-level thoughts.
Tabletop RPGs frequently involve "races" or "species" - types of characters who have some distinct background, shared across a large population group. Sometimes rather than being an actual biological race, they are just representations of cultural groups whose distinct cultures give characters advantages (e.g. Iron Kingdoms: Character Guide did this). I will refer to them as "races" despite this term being generally inaccurate for the overwhelming majority of such character options, because D&D did it that way.
In designing such races, there is a need to balance between legibility - the ability of players and GMs to take the information about the race and apply it to individual characters - and distinction - the fact that this race is actually different from other races in any meaningful way. If a setting has some source material that goes into details about the races of its world (e.g. Mass Effect) and the players can be assumed to have familiarity with it, the complexity allowed for legibility greatly increases, but for settings created for TTRPGs, whether original or published, legibility is greatly restricted in complexity. Nobody wants to do a ton of homework before creating their character or playing the game, after all.
To balance a race's distinctiveness and legibility, it is best to be able to describe the differences in terms of a few, large differences. Overall, I think there are four major ways a race can be made legibly distinct:
Mechanical Distinction
Mechanical distinction is when the race plays differently from the baseline (typically human) in a substantial way. Consider vampires, who are vulnerable to sunlight, or asari, who are all biotics. These are major, huge, influential traits that percolate through all characters of that race and cause them to make different decisions in the same situation (such as a vampire choosing not to go out because it is daytime). They are also best represented with game mechanics.
Like everything else, mechanical distinction works best when it is big and simple. A single, large trait (need to drink blood) is better than multiple smaller traits (proficiency with tools, resistance to poison, ability to see in the dark). Passive traits can also be difficult to remember when there are many and small, compared to when they are few and large (such as taking damage from the sun), which creates mental overhead for the player/GM (though it may depend on the specific game system and trait - +10 Strength is generally easy to remember). For weaknesses, it can also be relatively easy to keep in mind weaknesses that will largely only come up when an actor intentionally brings them into play (e.g. garlic for vampires or silver for werewolves).
Another thing to keep in mind with such mechanical distinctions is that it is generally better if these distinctions don't directly push certain character archetypes. In D&D, for example, it's better if a given race's bonus benefits fighter, cleric, and wizard characters, than if it only benefits fighters. Other RPGs have other subcategories of characters that you should think about instead, such as sneak/fight/talk/hack/know, or cybernetics versus psi, or the like.
The basic reason is twofold. Firstly, balance: a trait that's effectively worth 0 "points" if you're a Wizard but 6 "points" as a Fighter is difficult to balance for both character archetypes (really, only by creating an inverse ability that's valuable to Wizards but not to Fighters). Secondly, as a consequence, the race will be balanced as if the character was a Fighter, which means that it will suck to play that race but not be a Fighter, which reduces interesting choices for players in creating their characters.
Political Distinction
A race can be distinguished by its political position in the game world. The Quarians of Mass Effect, for example, are defined in the game's story not so much by their poor immune systems, as by their situation as exiles from their homeworld, forced to wander the galaxy.
Major types of such political distinctions include:
Model minority: Being rare but well-liked.
Hated minority: Being rare and disliked. Specific norms about how/why they are disliked matter too (if they're disliked for being thieves versus brutes versus diseased, for example).
Slave race: Being subjugated to another race, with the majority lacking basic freedoms or rights.
Client race: Being subjugated to another race's state, but having most (or all) rights.
Great power: Representing the dominant race of a top-tier political power.
Minor power: Representing the dominant race of a second-tier or below political power.
Details of the state a character is assumed to come from also have implications for the character. A character from a race who are a minor power on the outskirts of the major powers will be interpreted quite differently if that minor power is the Batarian Hegemony compared to if they are from the Chiss Ascendancy instead.
Overall, this is not about anything fundamental to the race, but instead entirely to the specifics of the setting itself. Imagine a Mass Effect where the Citadel Council was made up of the Krogan, Geth, and Batarians, while the Quarians were a client race of the Geth and the Turians were reeling from their attempted genocide by the Krogan. Even if all the other internal traits of the races - their government, their physiology, their appearance, their general character, etc - were kept the same, this would have a huge impact on how individual characters were perceived by others and how they act in the story, in a way that's easy to keep in your head.
Sociocultural
"Sociocultural" is used to refer to the ways that peoples are different in the real world, irrespective of the political regime they live under. Culture is basically a gigantic dumping ground, including things like diet, family organization, language, values, clothing, politeness norms, sexual morality, approach to law, etc.
Because cultures are so large and expansive, it is hard to cull them down to relatively simple "big points," and even relatively well-developed fictional cultures often end up as "planet of hats," such as the Vulcans and Klingons of Star Trek. This is mostly unavoidable, especially in the TTRPG context.
Some major tricks to use:
A major difference in values. A society that prizes innocence, cleverness, or bravery will be distinct from one that values honesty, fairness, or loyalty. These values often end up exaggerated (such as Klingon honor and Vulcan logic), but that's fine - a more exaggerated form helps make it both distinct and legible.
A real world referent, especially a version that is more the "pop culture" interpretation than the real one. This is why you see Space Romans and Fantasy Medieval England and the like so often.
Singular "big idea" which trickles down through the culture. If the culture is a surveillance society where psychic members form a superior nobility, there are a lot of things that naturally flow from that core idea.
Biological
Biological differences mean some type of basic physical difference between the races, with major downstream effects on behavior and psychology. The most prominent example is exceptionally long-lived races, since this has very little mechanical heft (most games don't care about age), while also implying lots of interesting things about characters of that race.
As always, fewer, bigger changes are easier to remember and roleplay. Yeerks are clearly distinct from the other races in their setting (Animorphs) due to the fact that they are slugs that have to infest another species' brain to move around on their own. In the same setting, the Taxxons' maddening hunger also gave them a very distinct characterization. The Kaylon of The Orville are distinct from the other major races due to being robots. A race that is eusocial, reproduces by mitosis, or has unusual environmental or dietary needs can also get distinction in this way.
This isn't something exclusive to science fiction settings, even though it's easier to remember there. In Eberron, Changelings and Warforged are quite distinct from one another and other races because of their biological differences: the ability to shapeshift in Changelings' case (with attendant effects on their culture and daily lives) and the lack of natural reproduction on the Warforged's part (with all sorts of implications for relationships and backstories).
Sometimes a point of distinction may overlap between multiple categories. Changelings' shapeshifting abilities carry both mechanical heft as their race's distinct power, and biological heft as something that impacts basic culture and daily life. This is good: it means that a single thing has gotten you extra distinctness without any additional cost in terms of legibility. The Yeerk of Animorphs are parasites (biological), have needs for kandrona rays and leaving their host's body on the regular (mechanical), and their nascent empire is premised on the need for hosts (political).