Snark was the only language we knew. It's like how Cassandra Caine was trained in the language of violence.
1) Cool, Individualistic Look.
You can't just a be a weird thing. You have to be appealing in one way to the other, be that looking badass, or attractive, or just fun. It also has to be customizable: a pallet of colors for hair skin and eyes -- notice how the gith are mostly grey and bald.
2) Powers
The species that have been grandfathered in via Tolkien get to be boring, but new folks can't. You have to do something interesting and fun that is applicable to most classes.
3) Open Lore
The trap of most failed species is that they get incredibly pigeon-holed in their lore in order to explain why they're not core races. They only live in this one place, they only have on moment that's important in their history, they inevitably have one culture. A species that's going to take off can mix into the rest of the world and fill multiple roles, not just 'sad monk' or 'wild man'.