My suggestion:
Level 1-10-Ur race starting zone
Level 10-23/24 Barrens or Ghostlands if you are Undead or Blood Elf
Level 23-31/32 Thousand Needles is my pick here
Level 31-41 I usually go stranglethorn but be careful,lots of pvpers there
Level 41-51 Tanaris is the way to go,really good and fast quests there+tonz of horde to help you out with questing
Level 51-58 Un'Goro crater,same as Tanaris,pretty good zone with fast quests
Level 58-64 Hellfire pen. or Zangarmarsh
Level 64-68 Nagrand-Ring of Blood quest there is a must!
Level 68-73-Borean Tundra
Level 73-77 You can go Dragonblight,Grizzly Hills or Zuldrak
Level 77-80 Scholazar is the way to go
Note:If you are stuck in some zone for whichever reason,you can always go and do couple of RDF-s,fast ques if you are tank or a healer