I found this helpful and hopefully you will too
0-15 - Questing fastest possible exp.
15-20 - The Barrens - Harpies *
15-20 - The Barrens - Bristleback(s) *
20-25 - The Barrens - Bael Dun Exavs *
20-25 - Hillsbrad - Hillsbrad Farmers (etc)
22-26 - Thousand Needles - Galak Scouts (etc) *
26-30 - Hillsbrad - Mud Gnolls *
25-30 - Thousand Needles - Grimtotems
30-38 - Shimmering Flats - All monsters. *
38-40 - Dustswallow Swamp - Very North East Islands, Murlock Warriors/Oracles.
Horde And Alliance Grinding Spots:
40-46 - Ferales - Woodpaws (stay away from the ones that disease for slow casting speeds) **
46-48 - Ferales - Frayfeather Skystormers *
48-50 - Ferales - Harpies
48-51 - Southwest of Gadgetzan - Thistleshrubs *
50-54 - Western Plaguelands - First 'field' to the left, assorted monsters.
54-60 - Western Plaguelands - Scarlet Lumberjacks
52-60 - Eastern Dire Maul - Lashers ****
* = Recommended
EXP Rates:
Levels 30-40 = 18-20k exp/hour
Levels 40-51 = 20-25k exp/hour
Levels 52-60 = 40-50k exp/hour