First of all when you hit level 20 you gain three very important things; your first mount, new abilities and the ability to enter the BattleGround Arathi Basin. These are all important in different ways. First, obviously new and improved abilities help you kill things faster and stay alive longer, which makes for faster wow power leveling. Of course your mount helps you move around the world of Azeroth faster which helps with questing.

For instance, you could do wow power leveling go from level 21-24 in about 3 games if you win. This is because the exp rewards at this level are higher than when you get into later levels. The reason I didn't mention Warsong Gulch is because in Arathi Basin, you get exp whether or not your team "capped the flag". In Warsong Gulch games are often pretty evenly split since you recieve exp throughout the whole game. Also, your team doesn't "give up" as much.

Also, whenever you do BG remember to pick up the 'daily' quest from Orgrimmar. This rewards you with extra honor, gold, and exp. It also boosts your reputation with Orgrimmar. But remember, Arathi Basin should be done sparingly, the best way to do wow power leveling is always through questing (unless you are a mage of course). But the daily quest makes it slightly more rewarding. An added perk of doing BGs is that you come out of them chock full of buffs which you can use for questing. In fact, when I know im going to have a hard fight I go into Warsong Gulch, get some buffs and /afk my way out. Cruel, I know, but the twinks can handle it.