Aerenal - I like the idea of the necromantic elven culture. It's almost enough to make elves cool.
Darguun - I really like the feeling of a land being pulled up from savagery to civilisation by the will of a single, powerful leader, and how fragile it all is.
Karrnath - A bit like Aerenal, the ambiguous role of necromancy is intriguing.
Lhazaar Principalities - Pirates are cool.
Q'Barra - Creepy jungles.