I just realized that other than the Romulans and Tholians, the editors/authors actually picked races we know very little about. We have the Breen, who despite being a major race in the end of DS9 we have never seen out of their suits, and who's government and homeworld we know nothing about. Then there's the Kinshaya, who have appeared in 2 books, and who we only learned about in ASD. And there's the Tzenkethi, who have only been referred to before. Oops... forgot about the Gorn, we do know quite a bit about them too.
I'm really hoping the TP miniseries will flesh out the other races more, because based on what little we know about the Tzenkethi, Kinshaya and Breen they do seem really cool.