Since the hits on the forum pwnt my board, I'm posting the guide here:
Alright, lots of people want to know the "correct" way to grind.. what to look for, where to level. I'm going to give my version.
Last update: 12.26.04
18-28 - Hillsbrad Foothills.Tons of kill quests, and a wide range of levels to fight. Also good for mining silk and greater healing potion reagents. Stay away from Alterac quests.
28-33 - Arathi Highlands.Few kill quests, but mostly good grinding stuff.
33-36 - Desolace.So many centuars... Watch out for useless quests. Best spot to farm grave moss (kodo graveyard). Stay away from the naga quests.
36-40 - Swamp of Sorrows.Great grinding/money spot, towards the end there are some decent kill quests along the far east coast. For the harder murlocks hit up the cave in the SE section. These levels will fly by
40-43 - Badlands.Very few quests, but enough stuff to grind very quickly off of. Rock elementals drop tons of stuff used in engineering/smithing and are also great to sell. I would suggest having a mount before coming to Badlands.
43-48 - Tanaris.Not many quests except for Zul Farrak (one of the elite instances w/5-6 quests, very worth running through), but tons of stuff to grind from. Rocs give the most loot to sell, ogres drop mageweave by the assloads, and the dogs go down easy as hell. Watch out for the useless "go find this" quests.
48-50 - Searing Gorge.If you are a frost mage, this will be heaven to you. Stay on the northern side of the zone to fight tons of quickly respawning golems. Also, lots of fire elementals for some of those hard to get fire items needed in crafting. No wind rider.
50-51 - Un-goro crater.This place blew. Do the kill quests, use Thott to find exactly where everything you need to kill spawns, and then leave. The quests give nice xp, but there's a much better place to grind off of which I'll mention next... You can get there from Tanaris. No wind rider.
52-54 - Western Plaguelands.For some reason, monsters here are incredibly easy... and drop runecloth :). I'm having fun grinding here now, will let you know when something new comes up. I don't think there's a wind rider, but it's really close to UC
54-?? - Burning Steppes.I've found a few mobs that go down easier than the easy ghouls at WP that were 5 levels lower. They are usually to the north. I'll let you guys explore yourself to find them. Again, this is an ideal place for a frost mage, as well as the staging ground for one of the all around best instances in the game for everyone: BRD. Because you'll be reaching about the end of your grinding here, go ahead and take some runs through the instance. The gear is well worth it.
It's important to note that a lot of these places can bleed over onto each other. Also, some of these places you can stay in longer and completely skip others... it all depends on if you want a change of scenery or not, and which monsters are easiest for you to kill. I would suggest trying them all out, but watch out for wasting too much time exploring places.
If you would like to add something, or would like to comment, please post here: Grinding Thread.