A professional overview
Alchemy Alchemy's main draw is that you can make your own consumables. Whether it's Flask of Titanic Strength or Flask of Steelskin, it's sort of nice to be able to know you'll have easier access to them than anyone. That said, it's easy enough to buy the flasks off the AH or get them from an alchemist friend or guildie.
That's where Mixology comes in. It gives a pretty sizable bonus to your flasks. Alchemy's last big draw is that you can make quite a bit of money through various transmutes.
Blacksmithing Blacksmithing remains a powerhouse in Cataclysm. In addition to giving you easy access to Ebonsteel Belt Buckles, it also gives you Socket Bracer and Socket Gloves. That's as much as 80 extra strength or 120 extra stamina from the gems you can socket in there.
Blacksmithing is also useful for providing yourself with armor and weapons as you level up. You'll be able to craft yourself, for example, some of the PvE epics we've linked in our gearing guides or a basic set of PvP gear if you want to break into that scene.
Enchanting Enchanting's big draw for the death knight is the ring enchants. Enchant Ring - Strength provides you with 80 extra strength when used on both rings, and Enchant Ring - Greater Stamina gives you up to 120 stamina. As you may notice, this essentially puts it on par with blacksmithing, although blacksmithing is arguably more versatile since you can slot other stats in the extra sockets as well.
In fun factor news, the Magic Lamp and Enchanted Lantern also come from enchanting, though they are BoE. Never underestimate the ease of being able to enchant your own gear, either, although that's probably a bit blunted now that scrolls are so easy to buy and use.
Engineering You have to be careful when you use engineering. Some items, such as the Cardboard Assassin or the Gnomish Army Knife or the Flexweave Underlay, have a value that can't be measured in DPS improvement. Other items, such as the Nitro Boosts, become a liability to use because a backfire is too costly. That said, at the least you'll get a dependable damage boost from the Tazik Shockers or a dependable armor boost from Quickflip Deflection Plates, which are more valuable than they first seem just because they do stack with other glove enchants. Come patch 4.0.6, engineering may even be on top of the heap for DPS death knights due to Synapse Springs, which will increase strength for death knights -- that's 480 strength for 12 seconds every minute.
One more advantage to engineering is the Reinforced Bio-Optic Killshades, which should last you until at least the first tier of Cataclysm raiding, thanks to their high stats and customization options. You can find cogwheels for just about any stat you could imagine at your faction's engineering vendor in Twilight Highlands. Don't get too attached your goggles, though. You'll probably need to replace them eventually, if only to get the set bonus on tier gear.
Herbalism Herbalism will provide you with Lifeblood, which will heal a bit of HP and provide 240 haste rating for 7 seconds at level 85. Haste rating is useful for some DPS specs admittedly, but this isn't as good as most options. Still, if you have the profession, it's definitely a button you'll want to push for some extra benefit.
Inscription Inscription isn't as dominant as it was in the Wrath era now that glyphs are one-time-use only, but you can still stand to make some good money, and it has a bunch of fun toys you can create as well. You can also make your own Dust of Disappearance, which considering how costly it is from vendors may come in more handy than you think if you have more ink than you can use on other things.
The biggest draw for being an Inscriptionist, however, is the shoulder enchant. Depending on which enchant you need, you'll get either 80 strength or 120 stamina over the Therazane exalted options, putting it on the same tier as blacksmithing and enchanting. Also, you can also avoid grinding Therazane reputation, which may be a grind for some people who are joyless drudges who do not love Pebble.
Jewelcrafting Jewelcrafting can be useful for making sure your gear is always gemmed at its best and for having easy access to a couple of very useful jewelry recipes. In addition to providing your own gems, you have access to Chimera's Eyes, which can provide up to 123 extra stamina or 81 extra strength in total over a non-jewelcrafter, as well as similar proportions of secondary stats. This actually gives it a slight extra stat edge over similar professions such as enchanting or inscription, albeit a negligible one.
Leatherworking Leatherworking is the surprising dark horse profession of the Cataclysm expansion. This is because not only do you get cheaper leg enchants, you also get bracer enchants that provide primary stats, enchants that don't even have any real analog for non-leatherworkers. Enchanters will get a strength bracer enchant in patch 4.0.6, but there's no enchant announcement or datamined for stamina currently. That means that for now, draconic embossments may be the best profession self-buff in game. You can get either 130 to strength or 195 to stamina from these embossments.
Mining Mining provides a straight-up 120 stamina buff, which can be a nice and simple bonus for a DK tank (if less versatile than the bonuses on other professions, which can be swapped or mixed and matched much easier). It's less optimal for DPS, but in Cataclysm, even DPS could use a little extra survivability, so it's not useless. Still, you'll probably only be taking this if you want to supply another profession or make money selling the stuff you harvest.
Skinning Skinning provides an 80 critical strike rating bonus. This bonus is near useless for tanks and only fair to middling for DPS, which means the only real reason for a death knight to take this skill is if you want to sell the skins for money or if you're leveling leatherworking.
Tailoring Tailoring does provide one useful thing for death knights. Swordguard Embroidery will increase your attack power by 1,000 for 15 seconds every so often. While a set-it-and-forget-it attack power proc is nice, it's still close to useless to tanks and far below what other professions get for DPS, especially once you factor in the loss of what other cloak enchant you could put on there. The only real reasons for you to be a tailor is if you want a flying carpet, want to be a fashion designer, or want to make your own bags.
Which professions should I get?
I have to admit, I get a little nitpicky when this question comes up. It's actually pretty open-ended. What you want is going to depend on what your goals are, what your resources are, what your guild needs, how much you care about what your guild needs, and what you find fun.
That said, I can give you a few pointers. If you're looking to absolutely min-max yourself, take two production trade skills that offer strength or stamina bonuses. Blacksmithing and jewelcrafting is still a very potent mixture here, both in power and versatility. Leatherworking also worms its way here due to the uniqueness of the bracer enchant, although again, it will be bought down to earth for DPS death knights in patch 4.0.6 due to the addition of the strength enchant. Inscription and enchanting also round out these choices, and engineering should claw its way back to the top of the pile for DPS in patch 4.0.6 when Synapse Springs get changed.
That said, don't be afraid to take a gathering secondary. It can be hard to keep up two production professions, especially if you don't have a lot of gold or a secondary gathering character at level 85. The extra gold you'll get from selling the surplus from your gathering skill can be an easy source of cash, especially if you're a religious reader of Gold Capped. Whatever way you go, good luck and good crafting.
Follow our road map to leveling your DK from 80 to 85, and then learn the ropes of endgame play with WoW Insider's DK 101 guide. Gear up with pre-heroic gear for DPS DKs and keep track of what's happening on your hotbars with our guide to DK spell alerts.