All of these areas have enough quests to easily get you to level 43
40-45: After you get your mount you have a few choices on where to level. You could go to aion gold Feralas, Strangle-thorn Vale or the Badlands.
All of these areas have enough quests to easily get you to level 43. At 43 head to Gadgetzan in Tanaris and make it your base. Load up on the quests here and when you are done you should be nearly 45.
There may be one or two yellow ones here so you may want to take them if you are not yet twenty. Either way, when you are done continue south and then left to Hilsbrad.
Follow the road until you get to Tarren Mill. Set this as your new Hearth-point and do the quests here.
45-50: Travel east to Steam-wheedle port and take on all the quests in the pirate base to the south. At level 47 head to Ashen-vale and go north into Felwood. Near the Emerald Circle camp is a Deadwood Furbolg base for you to grind on.
At forty eight take the quest from the friendly Furbolg NPC to collect feathers so you will achieve unfriendly reputation with Timber-maw Hold. When that has been completed head back to the Emerald Circle camp and take on their quests. This should get you close to forty nine.
Head south to marshals refuge in Ungoro Crater to finish off your level.
