Some of you may wonder, why?
Well, let's decide where your leading factor lies on choosing a profession. If you plan to raid, obviously you should aim for optimal performance, and use alternate characters to fund your raiding character. If you're playing the game for fun, pvp, or just casual monster bashing you'll probably care less about how much you'd benefit a group and want as much money with the least amount of effort.
Boosting your raid value
Leatherworking is currently the lead profession in regards to boosting raid performance. For two very simple reasons.
However, when WotLK is released the drums will cause a debuff on all affected players equal to the cooldown of the drums. This will render repeat effects useless. Meaning, instead of stacking four people per party with LW right now, you'll only want to use one person per party to have LW when WotLK is released.
Blizzard's reasons behind this are fairly obvious, current endgame guilds are noticing more and more the benefits of using drums, and almost every single player will end up as a leatherworker if this is not changed in the expansion. Blizzard has added a large number of pattern drops in the sunwell, for tailors, engineers, blacksmiths, and more. But, since people want / need to be leatherworker to boost their raid's DPS almost nobody wants those patterns and both blizzard as well as the players see this as an unwanted situation. However, they can't implement this "fix" straight away lest they cause a general uproar from the thousands of players who only recently maxed out their leatherworking skill. So, it will be changed in WotLK.
There will be new drums in WotLK as I posted earlier. And they will be powerful, but compared to what the profession can do for your raid right now, it'll be less of a requirement for all new raid members.
Enchanting is a good second for just about any class. 2x+4 to all stats to a rogue, warrior, druid or melee shamans will likely give them a noticeable boost in their DPS. Of course you may think professions such as blacksmithing or leatherworking (if you aren't already one) would give you awesome weapons or armor... Well, sorry to say but ring enchants can be used at any and all level of your game career. Weapons such as the 3 blacksmith tiers provide will only last you so long, and can be thrown away after that. On top of that, most crafted armor has equivalent versions available in one or another dungeon, even sunwell.
Hunters are semi-questionable as they don't really gain any benefit from strength. Still it will be hard to find a viable alternative to gaining +8 agi, +8 int, +8 stamina and +8 spirit by any other means.
For healers, and caster DPS there are healing and spell damage enchants. 2x12 spell damage, or 2x20 healing bonus. If you are a heavy potion user, you will want to seriously consider looking at alchemy however.
Jewelcrafting gives you two things, a trinket, and gems of slightly higher quality than most epic gems.
Gems which give : 24 AP, 18 stam, 12 spell crit, 12 hit rating, 14 spell damage, 26 healing, 12 spirit, 12 crit rating or 12 defence rating. Basically each gem can give you +2 extra stats in a single gem slot, the gems however are unique so you're only able to use each gem once in your armor. Most classes won't be able to find more than 3 gems which would actually benefit them.
Sunwell drops some patterns such as Hard Khorium Choker, however there is a fairly equal drop from brutallus called Clutch of Demise. The hard khorium choker is about a 7.36 DPS increase over the Clutch of demise. Enchanting both rings with +4 stats gives about a 10.89 DPS increase however. Swapping out two Glinting Pyrestones for a 24 and a 12 hit rating gem (the 12 crit one sucks) for a rogue would give 4.19 DPS extra.
You may have a hard time getting the sunwell pattern, but it's far from impossible. If you do you'd be looking at a DPS difference of : +7.36 from the neck, and +4.19 from the two gems, versus a +10.89 DPS from two rings enchanted. A total difference of +0,66 DPS as a Rogue jewelcrafter. Results may vary per class. PS : I used Rogue Raidcal for the calculations
On overall I'd recommend taking enchanting instead though. 8 more stamina, some dodge extra, and a profession you can use and re-use at any time in TBC and again in WotLK seems like a lot more value for your gold. If the DPS difference was bigger than 2, it might be worth considering.
Alchemist stones and mad alchemist potions. Unless you're a healer you're not going to get much benefit out of the stones. 40% extra mana is a big boost and will translate to about 60 mp/5 if you're chain chugging potions.
If you're looking for some cash then transmutations are a nice way to earn on the side.
With the arrival of the third and fourth arena season the tree "tier" weapon upgrades, as well as the early armorsmith plans are fairly redundant for any class. Sunwell has a few decent patterns for gloves or chest which some classes may be interested in, but for the most part this profession is indeed redundant for boosting your raid performance.
Frozen Shadowweave, Primal Mooncloth and Spellfire are the main movers in this profession at the lower levels. Mages, priests and warlocks will enjoy the craftable epics for a long time. Sunwell has a few decent chest piece patterns, as well as gloves. Much like blacksmithing and leatherworking however, there's nothing that doesn't have (fairly) equal items dropping from the same instance.
My als is a Mooncloth tailor, even though he's a warlock. Good for making bags :)
Engineering nowadays is mostly for making cash by farming primals, some trinkets and cables have the occasional use but otherwise it mostly falls short of expectations. It's a fun profession though. The sunwell head pieces are fairly good, the rogue headpiece is decent, but barely (if at all) better than what Illidan is able to provide you with. If your guild doesn't actively raid Black Temple you may wish to considder it. The pattern currently will cost you over 10k though. Also, Kil'Jaeden will drop a better head piece for pretty much all classes.
Unknown details at the time of writing this. Some incriptions will provide your spells and abilities with slightly reduced cast time, more damage or other modifiers. The inscriptions require crushed herbs / inks and will be available in WotLK.
There's a second part, indeed. Called Money making professions.
But it's coming at a later date ;) Stay tuned.