I just put this together to give the general idea on where to level at for Horde
1-10 Level were you start
10 Travel to Orgrimar
10-20 Level in the Barrens
20 Travel to Darkshire
20 - 30 Level in Darkshire area
30 Travel to Stranglethorn Vale
30 - 50 Level in Stranglethorn Vale make your way south as you Level
50 Travel to Deadwind Pass
50-60 Level in Deadwind pass -at level 58 go down near Karazhan and there are broken down houses and there are stairs leading down into a Cave there are level 69s there and there is great leveling there!