Gemming: Primordial Rubies are a death knight's best friend
For the most part, in gemming, you'll want to focus on getting your most important stat. That means you ignore gem slot bonuses unless they bring a whole lot of strength or haste (for DPS) or a whole lot of stamina or mastery (for tanks). This is both because strength is so far and away the best for DPS, but also because you can always reforge to get secondary stats, whereas getting main stats is a bit harder. It's also worth noting that the "Perfect" versions uncommon gem cuts are now rare, and now bolster your stats just as much as normal rare gems, so it may be worthing searching for those if you're gem shopping.
For DPS, you'll want to focus on strength, which means red gems. Bold Primordial Rubies are your rare choice. If you're just gemming some basic dungeon gear while waiting for raid gear, you can also try a Bold Pandarian Garnet. For yellow slots, look for fierce gems, either a Fierce Vermillion Onyx or a Fierce Tiger Opal. For a blue slot, go with an Etched Imperial Amethyst or an Etched Roguestone. For your metagem, a Reverberating Primal Diamond is tough to beat.
Gemming priorities are a little more nuanced for tank death knights. As we've discussed before, it's technically possible to have too much health, but it's not always easy to decide when the cut-off point is. Thus, you have a few options here.
If you feel like you don't have enough health, Go for a Solid River's Heart or a Solid Lapis Lazuli. If you do feel like you have enough health, focus on Fractured Sunstone or a Fractured Sun's Radiance. For slot bonuses, you can put a Puissant Alexandrite or a Puissant Wild Jade in a blue or yellow slot. For a red slot, go for Fine Vermillion Onyx, a Fine Tiger Opal, a Defender's Imperial Amethyst, or a Defender's Roguestone. For a tanking metagem, you'll want an Austere Primal Diamond.
If you are a jewelcrafter yourself, you'll be able to make your own stronger gems, called Serpent's Eyes. You'll make them with the same prefixes as above, but will only be able to use 2 at a time. You will, of course, want to focus on your primary stat for those gems.
Enchanting: From head to toe no longer
Head enchants are gone in Mists, so you no longer need to worry about those. In most slots, Mists has primarily only introduced one or two new options, so if you're really short on dough, you may need to consider digging up older enchants to make the difference. As with gems, your focus should be on primary stats where possible, as it's easier to reforge to get secondary stats.
For your shoulder enchants, you'll now be dependent on getting them from player scribes. Ox Horn Inscription or Greater Ox Horn Inscription will be your tanking choice, while Tiger Fang Inscription or Greater Tiger Fang Inscription will be your DPS choice.
If you're a scribe, the "secret" versions of the above enchants will be your choice.
For tanks, you'll want to try to Enchant Cloak - Greater Protection, while Enchant Cloak - Superior Critical Strike is your optimal DPS enchant.
Enchant Chest - Glorious Stats will be the DPS option, while Enchant Chest - Superior Stamina will be the tank option.
Enchant Bracer - Exceptional Strength is what you should be aiming for as a DPS. Note that tanks can also use this in order to get the parry conversion. Otherwise, they should aim for Enchant Bracer - Mastery. If you're a leatherworker, be sure to use a strength or stamina Fur Lining for this slot instead.
Gloves pretty much mirror bracers. You can get Enchant Gloves - Super Strength for DPS or tanks, or Enchant Gloves - Superior Mastery for tanks. Remember, enchants stack with any engineering glove attachments, so engineers, don't shirk on this slot.
While not en enchant per se, be sure to grab a Living Steel Belt Buckle and gem it with a gem in your primary stat.
You'll be buying your leg enhancements from leatherworkers once again. DPS should go with Brutal Leg Armor or Angerhide Leg Armor. Tanks will go with Toughened Leg Armor or Ironscale Leg Armor.
Unholy DPS will go for Enchant Boots - Greater Haste. Frost, which no longer has the movement speed increase of Unholy Presence, will want to consider Enchant Boots - Pandren's Step, which will also be the tank boot enchant of choice. Note that the run speed bonus will stack with Death's Advance.
While you can't normally enchant rings, if you are en enchanter yourself, you can. You'll want Greater Strength for DPS, Greater Stamina for tanks.
Our runeforges remain better than anything else out there as far as weapon enchantments go. Rune of Razorice will be on your main hand if you're frost DPS, with Rune of the Fallen Crusader on the offhand. Rune of the Fallen Crusader is also the choice for two-handed DPS death knights. Tank death knights will use Rune of Swordshattering.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.