Blizzard posted a few realms today that it suggests are low density. So if you are looking for a new place to start, check these out.

These realms are still very new and would be a great place to to create a character for new and seasoned players alike. They're in need of more people and offer a prime location to begin building a new guild, for a group of friends, or even just to meet some new people. Try a new class, race, or just be adventurous and come see what these realms have to offer!

Velen
Terokkar
Rivendare
Shu'halo
The Underbog