78 for the two starter zones, Hyjal and Vashj'ir. 82 for Deepholm, 83 for Uldum, 84(?) for Twilight Highlands.
According to WoWhead only 8 quests in Hyjal are available at 78, rest are 80, and not a single one in Vashj'ir is under 80
Hyjal seems to be the place you're looking for.