The Hinterlands & Searing Gorge (40-50): If there are any redwar gold quests remaining for these areas, they will mostly be around the low 50's.
Taking a group into Jintha'Alor to complete the quests there will get you some nice quest experience and rewards. The zones listed above are better suited for questing at this level area though.
Burning Steppes (50-58): To reach this zone, you much travel through the Searing Gorge (west of Badlands), and go through Blackrock Mountain. Horde has an outpost in the north of the zone, east of the Blackrock Mountain entry.
Alliance has an outpost in the Southeast corner of the zone. Both outposts have a flight path. It is not a very good zone for questing.
The number of quests that actually take place in this zone is limited; the majority takes place in the Blackrock Depths and Blackrock Spire instances.
If you are killing or questing in this zone, and see an elite giant named Volchan, he is soloable. Just use fear and do not let him get near you, and you're good. He drops green items most of the time. You can find dark iron ore in the thorium deposits here.