Here’s something I’ve learned, in my experience with 30 alts and all professions.
The only crafting professions you should powerlevel when you’re a low level are Alchemy, Enchanting, Tailoring from 150-210, and Engineering.
Edit 11/10/08 - Inscription is the new flavor for powerleveling. Get it up to the point of the Minor Inscription research, and just leave it there. You'll make way more off of the minor inscriptions that you can learn every 20 hours, than you'll make off the high level glyphs. I'm selling Aquatic form glyphs for 30-35 gold, and picking up major glyphs for a bid of 1 silver.
Enchanting so you can give better bonuses to your main and your alts equipment, and Engineering so you have access to higher damage bombs and equipment. There’s only a few Engineering items that have character level requirements (dragonlings, harvest reaper kits, guns & ammo..), so most of the high damage bombs/cool goggles are readily useable to your low level character when they possess a high level of engineering skill, if they’ve got the means to make them (high level alt, or buying mats off the AH).
Alchemy is nice to powerlevel, because it’s THE cheapest profession to pick up and max out. You definitely should powerlevel Alchemy at character level 35, so you can start doing Arcanite transmutes at lvl 275. It’s also very nice to have high level health potions, and the different buffing potions help you grind faster.
Edit 3/3/08: It’s probably more cost efficient to camp an alchemy trade supply vendor that has the health potions you want, and sell the mats on the AH. I know the level 21+ or 35+ health potion uses Sungrass, which is usually pretty expensive on the AH. For Allies, Southshore or Theramore has 21+ health potions, and for everyone, Winterspring and Darkmoon Fair has level 35 health potions.
With Tailoring, Leatherworking, Blacksmithing, Jewelcrafting….not so much. I powered thru to 225 with most of my alts on my Horde account, and most of the stuff I made I couldn’t use yet. So, it’s sitting in my bank, taking up space, waiting for me to level up the characters so they can use it. Worse yet, I’ve put decent enchants on most of the items, so I would feel bad about vendoring/DE’ing them. “Waste not, want not.” So with my new Ally account, I’m taking it easy with those crafting professions, and only leveling them when my characters need new gear. Your crafting materials and recipes could take up at least 2-5 bags of your available bank bag space if you had an alt collect the 1-300ish level recipes (see New Player tips), so you will only have the bank space to store items, unless you buy more bag slots.
Leatherworking is a prime example of this. It requires a ton of different mats, and the Leatherworking bags are too expensive for my blood at the current time.
(Tailoring)Update 11/19/07 - Tailoring might be powerleveled so that you can make White Bandit Masks at level 210. 1 bolt Mageweave + 1 bleach + 1 Heavy Silken Thread = $$$$$$. They actually changed the item in the new patch, so now it gives 11 intellect, and bonus spell damage and healing. Before, it was a cloth helm that gave Strength and Agility, which no melee person would wear. Anyway, see the New Player Tips for the reason why the White Bandit Mask is so good for disenchanting.
(Alchemy) Update 10/15/07 – I just powerleveled my lvl 22 Ally warlock from 149 Alchemy to 217 Alchemy last night, and cleaned out the trainer in Darnassus of all the recipes she can teach me. Make sure to check the recipes on Thottbot as you’re skilling up, so you can make potions that your other characters will use as mats in their professions. For example, the Elixir of Ogre’s strength will be used by two Jewelcrafting recipes, so make at least 10 to give to your JC for their future use. Healing potion is used by Truefaith Gloves, and Shadow Protection potion is used in Hands of Darkness, which are great Warlock/Shadow Priest gloves for lower level.
Avoid buying the Elixir of Fortitude recipe on the AH. It’s a Trainer taught recipe, so don’t pick it up. On the other hand, the Potion of Curing recipe (lvl 130) which can cure up to 4 levels of poison up lvl 60, IS a drop only recipe, yet it’s white, which usually signifies a vendor sold recipe. Go figure.
I love Oily Blackmouth Fish, by the way. You can use them to powerlevel Alchemy at lower levels with Blackmouth Oil, which you can use for Underwater Breathing Potion later, and at lvl 165+ use it for Free Action Potion. I say 165+ because I leveled from 149 to 167 using about 60 Firefin Snappers to make Fire oil (since it goes gray at 170). I made 20 Free Action Potions from all the Blackmouth oil I had saved, and that got me to 180+ I believe. The Free Action Potion doesn’t go gray until 205, so you could make a lot. It should sell for at least 50 silver to 1g a bottle, and it’ll stack up to 5 potions, so you could make a lot of gold off this recipe, since people buy it for Battlegrounds.
Get to fishing! It’ll bring you Oily Blackmouth, Firefin Snappers, a lot of Stranglekelp, and lockboxes pretty often. If you’re an avid enough fisherman, you could theoretically use it to gather all the leather needed for a profession like Engineering, without having to buy it off the AH. I came up from Theramore Island in Dustwallow Marsh with my lvl 22 Ally Warlock, and fishing in all the wreckage/Blackmouth pools from Theramore Keep onwards got me 40 Oily Blackmouth and about 7 lockboxes once I finished in the Ratchett harbor. I guess that Horde on Daggerspine must not fish a lot. :)
Update – 10/18/07 Horde Blacksmithing quests are funny. Why would they have you make silver bronzed boots and gauntlets AFTER you make Green Iron Helms? The Silver bronzed items are green, while the Green Iron items are orange. Make sure to make the items for Horns of Frenzy before Trampled Under Foot.