The key to fast leveling is maximizing your speed at killing enemies, while minimizing the time you have to spend eating/drinking to recover from battle. If you spec your druid properly and get the right gear, you can kill enemies your level in just a couple of seconds, then move on to the next one. Druids can achieve some of the highest DPS of any class in cat form with the right gear. The key is their stat benefits.
Classes benefit differently from different stats. If I remember correctly,
Warrior: 1 Str = 2 AP, 1 Agi = 0 AP
Rogue: 1 Str = 1 AP, 1 Agi = 1 AP
Druid: 1 Str = 2 AP, 1 Agi = 1 AP
Druids are the only class that gets the maximum benefits of attack power from both strength and agility. On top of this, by boosting agility, you gain critical strike chance, which is very beneficial to a feral druid in cat form. So you can maximize both critical strike and attack power at the same time.
Most green gear comes in many variations. For example you might find the same gloves can be found in "of Strength" (+5 Str), "of Agility" (+5 Agi), and "of the Tiger" (+4 Str AND +4 Agi). Of Strength will give you 10 AP. Of Agility will give you 5 AP and some crit. Of the Tiger will give you 12 AP and almost as much crit. By boosting both, you can get the most of your power and crit chance.
Because almost any green item can come in "of the Tiger" form, shopping for druid gear is very easy. Just check the Auction House each night and type "of the tiger" into the search field. It's somewhat rare so if you have the money, shop for yourself about 10 lvs ahead and nab everything that comes along until your whole gear is of the tiger. Of course if that isn't available, anything with strength and/or agility will be useful. Also straight up attack power can help you kill faster.
If you keep this up you will find yourself killing enemies extremely quickly and leveling fast.