The above poster is correct, then:
30-40ish - STV, and in also spend some time in Desolace. Personally I hate it there, but it is a nice change for a quest or 2.
40-48 or so. Tanaris and Azshara
43 - 48 - Also, Feralas
48 - 55 or so - Ungoro crater, Western Plaguelands
50ish - 56 or so - Felwood
55 plus - E plaguelands
58 - 60 Wintersprings and Silithus
Hope this helps a bit