Go back and look at the original descriptions of the races, from the 1E Player's Handbook and 1E Unearthed Arcana.
Then, consider the original, default conceptions of the other races, as presented in each subsequent expansion.
Then give them concrete, major powers, that make it possible for them to actually be like that.
Elves: Are immune to all natural injury, poison, and other effects. Rarely need to eat or drink, may eat and drink very exotic things (in other words, they do not farm.)
Dwarves: Do not need to breath normal air. Can breathe in pure carbon dioxide, even in carbon monoxide. Immune to toxic fumes, immune to most toxic chemicals. May see normally in complete darkness.
Gnomes: Immune to natural dangers as per elves. Immune to chemical poisoning as per dwarves. But do need to breathe normal air and eat normal foodstuffs.
Halflings: Supernatural constitution with all the bonuses, have most of the abilities of the Rogue as natural abilities.
Half-Orcs: Tremendous strength and consitution, able to eat anything, able to subsist in horrific environments, virtually immune to all poisons and infections, ability to Berserk like a barbarian.
Kender: Immune to normal and magical fear and awe, all of the abilities of the Rogue except backstab, unsconsciously steal constantly, intense curiosity at all times.
The People of Athas: May start with 24s in their stats.
Humans: Are the dominant race. Have major advantages of some sort, within the rules, that make them the most attractive race to play, make them more powerful than everyone else.
Perhaps humans can Gestalt, while all others must use the 3E multiclassing rules.
Perhaps humans gain an extra feat (or an extra two or three feats) each level.
Perhaps humans can be classes nobody else can be.
Perhaps humans can reach levels nobody else can.
Whatever the case, You Take on Humanity at Your Peril.