Since I've been playing a lot with all the professions, I'll try my hand at guides to the professions. Any input is welcome, and I'll edit and update them as necessary.
This first part of the guide assumes that you are choosing what professions to take on a brand new character with no high-level support, and that you don't have much experience with professions, if any. I organize the typical starting options into the Best Options, which are profitable for leveling players, Very Good Options, which are very profitable in the long run and fairly easy to level, and Good Options, which can be a short term time and gold cost, but are still good in the long run. I also list Other Options and why I don't recommend them.
I use WoW-Professions as a general guide to leveling professions. I highly recommend using Auctionator to buy and sell on the Auction House and something like Postal for the mailbox, since the default user interface is just too slow and clumsy.
The Best Options
Herbalism/MiningMy highest recommendation for any player first starting off. Dual gathering goes along well with leveling, as it doesn't take time away from actually leveling.
- It's bonuses are survival-oriented (Lifeblood from Herbalism as soon as you take the profession, Toughness from Mining at 75 points), and they scale as you level.
- They don't cost anything to level but a nominal fee for trainin.
- Gathering herbs and ore grants a decent amount of experience.
I recommend selling herbs and ore on the AH in stacks of 5, 10, or 20. This gives Inscribers and Jewelcrafters convenient stacks for milling and prospecting. Ore can also be sold in stacks of 1 or 2, for Miners to smelt.
WoW-Profession's guides to Herbalism and Mining are intended to be as efficient as possible for a player at max level, so they may not be the best suited for a player while leveling. If either profession lags behind, though, following these is a great way to catch up.
Herbalism/InscriptionInscription is uniquely profitable very early on (My low-level Inscription guide), and is great for any class just starting off. Be aware that the glyph market can be very competitive, but there's a lot of profit for the dedicated. Just resist the urge to camp, which is canceling and re-listing auctions that get undercut. That's a huge time sink.
Very Good Options
Mining/JewelcraftingAny class can benefit from Jewelcrafting, and it has some nice bonuses while leveling, including Stone Statues that heal you, access to some nice rings and neckpieces very early on, and trinkets in the mid-levels and onward.
Leveling Jewelcrafting along with Mining is, of course, the most economical method. Take this option of you are considering being a serious Gold Maker later on, as Jewelcrafting is a very profitable profession on it's own, as well as being the first half of the Ore Shuffle.
Tailoring/EnchantingTailoring is a common choice for Priests, Warlocks, and Mages, so they can craft their own armor. There is certainly merit to this in the endgame, since being able to craft armor using your own material and professions can be a great money saver in the long run. While leveling up to 90, though, Tailoring does not actually provide much benefit at all. There are plenty enough great quest rewards and dungeon drops that most items you will be able to craft won't be an upgrade. Even in the few cases where there are upgrades, it's replaced within a few levels.
Tailoring is classically paired with Enchanting, since crafted items from Tailoring can be used to level Enchanting. Enchanting also does not provide much benefit while leveling, since putting enchants on leveling gear is largely a waste of time and material. The major advantage to having Enchanting while leveling enables a player to always be able to disenchant gear drops in a dungeon and many quest rewards for more than they vendor for.
Take this option if you think you want to be a Tailor and/or Enchanter when you get to max level. Enchanting is the second half of the Ore Shuffle, so it's also a great choice for anyone who is considering being a Gold Maker.
Skinning/LeatherworkingThis is a very popular choice for the Leather and Mail wearing classes. Leveling Leatherworking without having Skinning to support is is famously difficult and frustrating, and leveling Skinning without Leatherworking is just about useless, so the two are best leveled together.
Much like Tailoring, Leatherworking also does not provide much benefit while leveling. Skinning does provide a small Crit bonus which scales as you level, but does not give an XP bonus, as Herbalism and Mining do.
Take this option only if you are sure you want to be a Leatherworker at max level.
Mining/BlacksmithingPopular for Warriors, Paladins, and Death Knights, Blacksmithing also does not provide much benefit while leveling.
Take this option if you are sure you want to be a Blacksmith at max level, but be aware that Blacksmithing will use up most of the ore you gather, which cuts into any profit you would make from Mining.
Herbalism/AlchemyWhile leveling, Alchemy yields mostly useless items. Typically, enough potions drop for players along the way that anything you can make is usually excess. Also, like Blacksmithing and Mining, Alchemy cuts into Herbalism's potential profits.
Take this option if you are sure you want to be an Alchemist at max level. It's a very profitable profession at max level.
Mining/EngineeringEngineering is really fun, with lot's of cool toys. It's especially great for leveling in battlegrounds, where it pairs well with Mining's Toughness.
At max level, most crafted Engineering items are only useful to the Engineer, but between guns, scopes, pets, and mounts, it has a few niche markets for the dedicated.
Take this option only if you are dead set on being an Engineer. It's an expensive hassle to level, and not the most profitable in the long run, but it's fun!
Other OptionsAny combination of two crafting professions, i.e. Jewelcrafting/Blacksmithing, is not good for leveling characters who don't have thousands of gold to invest in these professions, as well as the time and patience to use the AH for materials.
Skinning along with any profession other than Leatherworking is largely useless, unless you are planning on being a double farmer at max level. In that case, it's OK to pair with Herbalism or Mining.
Enchanting paired with a crafting professions other than Tailoring suffers the disadvantage of being unable to gather materials for crafting.