So, unlike a lot of people, I actually do a certain amount of farming in part of my gold-making system. This is mainly because I still don't have a ton of capital, but it's also because I actually kind of like farming.

One thing I often farm for myself is Essence of Air. This is one of the elemental items from Classic WoW and is used in two major heirloom/twink enchanting recipes: Enchant Weapon - Agility (rogues, enhancement shamans, hunters) and Enchant 2H Weapon - Agility (cat and bear feral druids). Both of these recipes come from the Timbermaw Furbolg faction, and only require Honored reputation to get.

Back before I had an enchanter who could make these recipes, I often farmed Essence of Air and just sold it on the AH. On my server it can sell from anywhere from 60g to 125g per.

The reason it sells for so much is because it is kind of a pain to farm. The best place for Essence of Air is in The Crystal Vale in Silithus. This is a 55-58 zone. The mobs that drop the item are called Dust Stormers. They are around level 55. (If you are leveling 55-58 in Silithus, you'll want to know that a type of mob called Cyclone Warrior found further south in the zone also drops Essence of Air.)

A map of my farming location. I ignore the zone full of Earth elementals, and farm the air elementals and thorium

Before the Shattering, there would occasionally be a small elemental invasion event here on a semi-regular basis, and those mobs dropped Essence of Air at a much greater rate. However, that has been removed from the game, and now we are left with the air elementals that are always there. Farming for 30 minutes (the length of a Seal) usually nets me around 6 essences (I've gotten as few as 4 and as many as 10 in this time), but like all random drops, it's very RNG dependent.

However, keep in mind that I am not always farming the air elementals there during that time. While I'm in this location, I also mine every thorium node I see.

A short breakdown of what I might do with spoils from mining thorium:
uses: Alchemy, Blacksmithing, Enchanting, Mining. Can also benefit from, but does not require: Skinning

1. Arcane Crystals I mined from Thorium nodes are sent with an equal number of Thorium Bars to my transmute spec alchemist to create Arcanite Bar. Arcanite Bar can be sold as is, or combined with Dense Stone by a Blacksmith to make Arcanite Rod.

2. Simple gems (Azerothian Diamond, Huge Emerald, Large Opal, etc.) looted from nodes mined are put on the AH for leveling jewelcrafters to buy and use in craftable jewelery.

3. Dense stone is sent to Blacksmith to be turned into Dense Grinding Stone. These are either combined with Arcanite Bar to make Arcanite Rod (see above), or can be turned in at the Darkmoon Faire for tickets to redeem for a chance at rare crafting patterns.

4. If I have more than 2-3 stacks of ore, at least half of the remaining thorium is smelted into bars and sent to Blacksmith. Blacksmith crafts Enchanted Thorium Blades. (This recipe also requires Enchanted Thorium Bar and Rugged Leather, which I usually just buy on the AH. If Dream Dust is too expensive to make your own Enchanted thorium, you can buy the 2 bars needed for each blade on the AH. Since making Enchanted Thorium can be used to level Enchanting, the bars are often available for cheaper than the raw mats.) These are then disenchanted into Greater Eternal Essence, or occasionally Illusion Dust, or more rarely a Large Brilliant Shard. These are either sold directly on the AH or used in heirloom/twink enchanting scrolls. (This is actually a fairly decent method for gold neutral to gold positive leveling of Blacksmithing from 300-340, by the way. This also allows you to almost entirely skip Outland mats.)

5. The remaining thorium is either smelted into bars or left as ore and sold on the AH. If I have a great deal of Thorium, I sell some as bars and some as ore simultaneously.
I can confirm (I looted it yesterday) that Formula: Enchant Weapon - Crusader is back in the game as of patch 4.1. It drops off of Scarlet Archmages in the Tyr's Hand region of Eastern Plaguelands. These mobs are no longer there if you have done the Tyr's Hand parts of the Eastern Plaguelands questchain, which means you won't be able to farm this on your achievement toon. The drop rate is still pretty low, so be prepared to settle in for the long haul if you're farming this yourself.

People seem to be paying a slightly inflated price for the formula still. (Some people don't know it's back in the game, I guess.) So while I went out to farm this for myself, I ended up selling the formula instead of learning it.
I love Azuremyst Isle. This is partially because my first character ever was a Draenei, so it's sort of the place where I know where everything is. I spent weeks slowly leveling through all the Azuremyst/Bloodmyst content when I was a wee noob, so it's very calming and homelike to me. I mean, for goodness' sake, I keep my bank alt in the Exodar.

Today, I decided to work on leveling herbalism and skinning on a human rogue character I have, and I decided to do this on Azuremyst. It's not just that I love it, it's that it's a genuinely good place to do this. Why? There's a lot of cool stuff on Azuremyst Isle.

Low Population Pretty much the only people you are going to run into in Azuremyst Isle are lowbie draenei characters. That means you're not going to run into much farming competition. If your server isn't high pop, there is a pretty good chance that you will be the only person in the zone.

Holy cow, Draenei men are huge.

Recipe: Roasted Moongraze Tenderloin One of the few Alliance quest recipes that is not also sold by Kendor Kabonka. It comes from The Great Moongraze Hunt, a level 6 quest with no pre-reqs. I make a point of bringing my Alliance toons to Azuremyst at least once to do this quest, so I can sell the recipe to the Horde. (Even if you don't have a system in place to sell to the Horde, this recipe will eventually sell on the Neutral AH.)

Small Eggs The best drop rates for Small Eggs are in Eversong Woods, which is, for obvious reasons, easier to farm if you're Horde. For less adventuresome Alliance, Azuremyst has two great sources of Small Eggs. If you're leveling skinning, the various Timberstriders don't have the greatest drop rate, but are skinnable, unlike the vultures in Westfall. (The other good source is the moonkin on Silvermyst, who have a better drop rate than the vultures in Westfall.) Two times out of the year (Children's Week and Winter Veil), Small Eggs become incredibly expensive, and are worth stocking up on.

Blood Elf Bandit Mask A popular white drop from the Blood Elf Bandit NPC found on Azuremyst. I wouldn't farm it, since he's hard to find/not always up, and the drop rate is pretty low, but set up a targeting macro (since he's stealthed), and mash it when you run past a spawn point, just in case.