Druid is a very efficient leveler with very low levels of downtime due to its ability to deal damage with energy and rage in
Cat Form and
Bear Form respectively while simultaneously regenerating mana that keeps you healthy via powerful HoTs like
Regrowth and
Rejuvenation.
In Forever
Balance received large buffs and is now a theoretically viable option for leveling (and will be better at healing dungeons while leveling) however given that we cant test 30-60 exhaustively before launch I would recommend opting for the known quantity in Feral.