Well ive been lvling my protadin pretty quickly, and gearing him up really well, hes 50 now, has a Hanzo Sword with crusader and a nice shield.
So heres just a few spots i found useful.
30-34 = Shimmering Flats, theres a few spots by wreckages where the mobs are tightly packed together and an easy AOE spot, scorpions, vultures, turtles and basilisks.
34-37 = Outside RFD, theres a few casters and hte knockdown can get annoying sometimes, but if your playing your protadin right you shouldent be healing yourself.
35-36 = Ogres in Mid-Eastern Alterac Mountains, easy kills, no special abilities, all melee, plenty to group pull and a rarespawn that goes down no problem.
37-38 = Murlocks in Theramoors, if your waiting for more to respawn, next to jarl's farm is plenty of undead melee mobs, take a break and take a few quickly down with exorcism.
38-40 = Run RFD and SM alot, great gear, quests. Also a nice spot was the animals in STV, theres plenty to go around, raptors were my favorite, but disarm was annoying, reccomended weapon chain if you want to farm them. Also, theramoore has plenty of towers where you can pull the entire tower and AOE em down no problem.
40-42 = Wastewander Bandits/Thieves, usually you can get max 3 of em, not the best spot, one i liked slightly more was the dwarves outside uldaman, the casters dont hhurt too much and the other dwarves go down quick.
43-45 = Zul Farrak , you should be able to tank it at these lvls, each run should give you about 60% of your bar to lvl, also the trolls in hinterlands (noncasters) arent too bad either.
45-46 = Pirates in tanaris, we all know that spot.
47-48 = Here people have a conflict with me, i LOVE the ogres in tanaris, all duel wielders and their special attack just knocks you into the air for a few seconds. the Dunemaul Compound has all melee around the outside , easy to kill ogres and the casters dont hit too hard either, i highly reccomend this spot , found an epic here and 2 rares.
48-49 = I Chose to run maraudon for these lvls, since thrash blade is pretty nice, theres also a few nice drops.
49-50 (where im currently at) = Heres my favorite spot in the game, Sorrow Hill, crapload of mobs, quick respawn, you get marks, and even without the commission, i got to honored in about a day farming here.
50-51 = Planning to run ST a few times, nice shield and swords in there.
51-53 = More WPL, the different farms, and sorrow hill should still be really nice for you.
53-55 = Badlands ive heard has some nice spots, the netherguard miners, and overall all the animals, not to mention the ogres.
55-56 = Should try to run BRD, a full run will easily get you a lvl maybe 2, and theres some insane gear in here, reccomended you get Force of Will.
56-57 = Yetis in tanaris ive heard are a popular spot, you can also try the higher parts of WPL or some EPL.
57-58 = Now your in outlands, enjoy.
Forgot to add, HIGHLY RECCOMENDED ITEMS
Truesilver Breastplate
The Green Tower
Firey enchant for your sword (deffinately on sword of omen)
Thorium Shield spike on your shield.
Uther's Will (ONE OF MY FAVORITES, procs easily 5-6 times durring a pull of 5+)