Mining and blacksmithing
Peanut butter and jelly. Ashford and Simpson. Mining and blacksmithing. Somethings were just meant to go together. Unfortunately, though, the synergy between mining and blacksmithing does little for a shadow priest. (Don't worry, though -- Ashford and Simpson are still solid.)
Don't get me wrong, mining is still a great way to earn some quick cash (if you have the patience to farm, that is). The problem is that it's terrible in terms of endgame buffs. At mining 525, you'll get a 120-point boost to stamina via Toughness. That translates to a 1,680 increase in your maximum health -- an almost negligible bonus for a shadow priest. It's the only profession that won't boost your DPS.
Blacksmithing is marginally better, at least in terms of buffing your DPS -- blacksmiths get to add an extra gem slot to their bracers and gloves. If you fill both of those prismatic slots with Brilliant Inferno Rubies, you'll get an 80-point intellect bonus from the profession. Aside from that (and possibly the ability to craft and sell Ebonsteel Belt Buckles), there's not much benefit to a profession that crafts gear a priest cannot use. Save blacksmithing and mining for your paladin, warrior, or death knight.
Skinning and leatherworking
Leatherworking and skinning are another pair of professions with tremendous synergy. But like mining and blacksmithing, that synergy is lost on shadow priests.
For shadow priests, skinning is purely a money-making profession. Leather does sell, and you can make some good money by doing little more than regular questing and running heroics. The inherent problem with skinning is that its profession-specific buff is one of the weakest in the game: an 80-point buff to critical strike via Master of Anatomy (requires skinning 525). As of patch 4.0.3, that's worth somewhere around 25 points worth of intellect (or pseudo-intellect, anyway); in patch 4.0.6, crit's value won't be much better.
The endgame buff from leatherworking is much more competitive; at leatherworking 500, you have access to Draconic Embossment - Intellect, a 130-point intellect bracer enchant. The best non-leatherworker bracer enchant for shadow priests is 65 haste, which translates to about 26 intellect. That makes leatherworking a surprisingly competitive profession for us -- it's worth 106 intellect.
The downside to taking leatherworking, of course, is obvious: Priests can't wear leather gear. There isn't much else to do with all that unusable gear but sell it to other players (often at a significant monetary loss).
In terms of gathering professions, herbalism is one of my favorites. Herbs are used in both alchemy and inscription. Since every class has need for potions, flasks, and glyphs, herbs will always be in demand.
In Wrath, the profession-only bonus for herbalism was a weak heal over time spell. Granted, it was castable in shadowform, but being able to heal for ~4,000 health over 5 seconds didn't do anything to help your DPS numbers. Thankfully, the buff Lifeblood was redesigned for Cataclysm and now provides an on-use haste-plus-heals effect, instead of an on-use heal. The heal is weak, but the haste boost at herbalism 525 is significant: 480 points for 20 seconds. Given that Lifeblood is on a 2-minute cooldown, that averages to 80 haste (worth about 32 points of intellect).
Herbalism has a secondary profession-only benefit: the Lifegiving Seed. It's a random drop off herb nodes that, on use, turns you into a plant. While you're disguised as a plant, you restore large amounts of health and mana quickly (though sadly, not in combat). It's a nice convenience, but not enough of one to make up for the weak 80 haste profession bonus.
When I first started playing World of Warcraft, all my friends told me that alchemy was a great money-maker. I didn't really appreciate why or how until I finally leveled an alchemist myself. Being able to make your own potions and flasks is great, but having a once-per-day "transmute" is where the real money was. In Wrath, I'd convert rare gems to epic gems. In Cataclysm, I convert volatile elements into Truegold. It's free money every day.
Of course, there's a benefit to alchemy beyond the money. Alchemists see an increased effect and duration from flasks and potions. Drink a Flask of the Draconic Mind, and you'll get an extra 80 intellect and an extra hour worth of duration. Or for alchemists on a budget, there's always the non-consumable, bind on pickup Flask of Enhancement. It provides an 80-point intellect buff any time you want.
Once patch 4.0.6 goes live, alchemists will have access to one more profession-specific perk at alchemy 500: the Vibrant Alchemist Stone. An ilevel 359 trinket, the Vibrant Alchemist Stone will provide 301 intellect, a red socket (with +10 intellect bonus), and 194 points of haste. And if that wasn't enough, the stone will also increase the effect of healing and mana potions by 40%. It won't be the best DPS trinket in the game (or even the best craftable trinket), but it's still one heck of a nice perk nonetheless.
Tailoring, the art of converting those scraps of Embersilk Cloth you find on humanoid enemies into wearable garments, is something of a priest mainstay. The reasons are obvious -- priests are a cloth-wearing class, and by choosing tailoring, you're able to fill virtually every gear slot with well-itemized ilevel 333 and 339s (and sometimes 359 gear, like the Breeches of Mended Nightmares). Tailors are also the only profession capable of crafting leg "enchants."
That's not all there is to tailoring, of course. Tailors get access to the best leg enchants (plus 95 intellect and 55 spirit) without having to use Dreamcloth. That's a huge money-saver at this point in the game, since a piece of Dreamcloth can cost 1,500 gold to craft. The real tailor-only benefit, however, is in Lightweave Embroidery, a tailor-only cloak enchant. The proc, 580 intellect for 15 seconds, is one of the best profession-specific benefits in the game.
The proc itself lasts for 15 seconds, and there's a 45-second cooldown before Lightweave can re-proc. If it takes about 5 seconds' worth of casting to proc; that means you're getting, on average, 580 * (15/50) = 174 intellect. That's a whole 124 intellect better than Greater Intellect, the shadow priest's cloak enchant of second resort. And best of all, because you get all that 580 intellect at once (instead of averaged out over a given fight), you can time your shadowfiend or Dark Archangel abilities to coincide with the proc for a beautiful multipier effect.
Enchanting is a pretty straightforward profession. You "dismantle" green-quality or better gear, get their magical essences (like Hypnotic Dust and Heavenly Shards), and use them to improve the stats of gear that you actually want to wear. Enchanters are the only ones who can "disenchant," so by choosing enchanting, you'll have much greater access to enchanting materials than you'd otherwise have. It's a solid (if passive) money-maker -- the disenchanted materials are usually worth more than the vendor price for the gear.
Enchanters are the only people in the game capable of having ring enchants. For the cost of one Heavenly Shard, you can increase your intellect by 40. Multiplied across the two rings you'll have equipped, enchanting provides an 80 intellect bonus, accessible as soon as you hit enchanting 475 (and level 75).
In Wrath, inscription was all about glyphs. With few new glyphs added for Cataclysm, inscription has changed into something of a boutique profession for the "fun" and "unusual." Shadow priests will be able to take advantage of a number of scribe-created items: Scroll of Intellect IX (a 100-intellect buff that's great for heroics); all the priest-related glyphs you want at a relative bargain; some nice off-hands for leveling (like the ilevel 346 Divine Companion); and one of the best shadow priest trinkets available, the ilevel 359 Darkmoon Card: Volcano (via Darkmoon cards). Inscription is also a tremendous money-maker, given a steady supply of affordable herbs, as there will always be a market for those damn Mysterious Fortune Cards.
All the above craftables are great, of course, but you can buy each and every of the above off a scribe, often for less than the cost of materials. What you can't buy is the scribe-only bonus, Felfire Inscription. It's a shoulder enchant available at inscription 500 that will boost intellect by 130 and haste by 25. At a cost of four Blackfallow Inks, you'll be able to get 80 more intellect than the Greater Inscription of Charged Lodestone, which requires you to be exalted with Therazane.
Though none of my characters in World of Warcraft are engineers, I'm still something of an expert on the subject. You see, I graduated from Rutgers with a degree in chemical engineering. And if my experience there is any judge, engineering is all about reading Dilbert, playing bootleg copies of Super Puzzle Fighter until 4 a.m. in the morning with your Korean roommate, and being told you'll get a $100,000-a-year job only to find out, once you graduate, that no one is hiring.
Oh, and blowing stuff up. Engineering is all about blowing stuff up. So, it should come as no surprise that engineers have access to a profession-only buff that helps them blow stuff up in a more efficient manner. Engineers get access to the Synapse Springs, an on-use, 480 intellect buff that lasts 12 seconds. It has a cooldown of 60 seconds, so the buff currently averages out to 96 intellect. (This is being nerfed in patch 4.0.6 to last only 10 seconds, which will cut the buff down to the standard 80 intellect.) Having this tinker is not mutually exclusive with standard glove enchants -- you can have both.
The value of engineering goes beyond those 80 points of intellect, though. Engineers still get access to a repair bot, and they still get access to a portable mailbox. Better yet (from a DPS perspective, at least), engineers -- and only engineers -- get access to the Lightweight Bio-Optic Killshades, an epic ilevel 359 head piece. And best of all, engineers can choose the two specific secondary stats they want on the gear: 208 points worth of haste, crit, mastery, or spirit, whichever is desired the most. It's an expensive piece of equipment to craft, but if you're hungry for stronger DPS, it's absolutely worth every copper you'll spend.
As a benefit from jewelcrafting (500), you can create the jeweler-only Brilliant Chimera's Eye, worth 67 intellect. That's 27 intellect more than the Brilliant Inferno Ruby (the best intellect gem for us non-jewelers). Since jewelcrafters can equip a maximum of three Chimera's Eye gems, that's a benefit of 27 x 3 = 81 intellect. One whole point of intellect better than the standard plus 80.
One single point of intellect over the other professions isn't make-or-break. What helps put jewelcrafting over the edge are the jeweler-only trinkets that the profession has available. The Figurine - Jeweled Serpent is a craftable, bind on pickup, ilevel 346 that provides a massive 285 points of intellect with a 1,425 spellpower on-use proc. You can only have one, and you have to get a world drop quest item to craft it, but the drop rate is high enough that it should take no more than a day of solid questing to find.
At patch 4.0.3 values, the Jeweled Serpent clocks in at over 451 pseudo-intellect. That makes it roughly comparable to the value of the Witching Hourglass as a best-in-slot, pre-raiding trinket.
And the award for Best Profession in Cataclysm goes to ...
I'm going to cop out here and say something lame: The best professions are the ones that work the best for your individual goals. If you want to do nothing but make money, I really like the combination of inscription and alchemy. If you're a strict min-max shadow priest, tailoring and jewelcrafting stand out.
As to which is best ... well, that all depends on how good you are at rolling for loot during your raids. That engineering helm could be the best thing ever, far surpassing the slight intellect buff you'd get by choosing tailoring instead. Or you could win a roll for the Crown of the Twilight Queen next week and wind up with a 10 gold, 51 silver piece of vendor trash that cost you a hundred times that to make. And heck, if you're dutiful enough with your mining node searching, you could just buy the Je'Tze's Sparkling Tiara off the auction house via mining. Professions are what you make of them.
So, in the long run, there's no specific right answer. (I will continue to insist, though, there's a definite wrong answer, and mining is it.) In review, the class-specific buffs/benefits are (as of patch 4.0.6):
- Tailoring ~124 intellect; cheap leg enchants; you find more Embersilk than usual
- Leatherworking ~106 intellect
- Jewelcrafting 81 intellect; best pre-raid trinket
- Engineering 80 intellect; perfectly itemized epic helms
- Alchemy 80 intellect; good pre-raid trinket
- Inscription 80 intellect; freedom from the Therazane grind
- Enchanting 80 intellect; ability to disenchant old gear
- Blacksmithing 80 intellect
- Herbalism ~32 intellect; minor heal
- Skinning ~25 intellect; the good feeling you get being nature's janitor
- Mining no intellect; 120 stamina
Are you more interested in watching health bars go down than watching them bounce back up? Think it's neat to dissolve into a ball of pure shadow every few minutes? Hunger for the tangy flesh of gnomes? The darker, shadowy side of Spiritual Guidance has you covered (occasionally through the use of puppets).