Shoulders: Do Regular/Heroic HoS and get the quest reward shoulders from inside the instance. They'll work until you can get better ones. I forget what they're called but the quest giver is Brann Bronzebeard.
Gloves: You bought the DPS ones, which is fine since you're dual-spec. The tank gloves are Gauntlets of the Royal Watch. Buy and equip ASAP. Yor enchant is right though, especially tanking with 3k+ DPS.
Cloak: Emblems of Valor = Platinum Mesh Cloak. That, or run H Nexus, kill Kolurg, and pick up Cloak of the Enemy. Both nice tank cloaks pre-naxx.
Sigil: Once you get def capped, switch to Sigil of Deflection for more rune strikes, which means better single target threat.
Trinket: The Black Heart is excellent, but the Brawler's Souveneir needs to go. Until you're capped, run reg HoL and get Seal of the Pantheon. After you cap, switch to Essence of Gossamer for more stam UNLESS you have one of the brew charm trinkets. 170 stam is nothing to sneeze at!
Ring: Emblems of Valor = Signet of the Impregnable Fortress. It's an ilvl downgrade (213) but item level matters a hell of a lot less than being able to do your job successfully without being a mana sponge and that ring has def rate.
Once you get capped, switch out any orange gems for str/hit when you have the room to do so without going under cap. Also, you need expertise pretty bad, so that's another use for those red sockets until you get some better stuff.
As far as managing threat goes, all I can suggest is mark your targets and stick to those marks. Also, make sure the DPS gives you enough of a head start to hold aggro through the fights. I use Unholy to DPS, so I'm not sure how the threat management goes in that spec. Most of all, stick with it! If you're already downing raid content successfully without having 540 def skill, imagine how much easier it'll be without watching those incoming 20-25k crits!
...Thoughts of a philosophical Death Knight...
If death is the inevitable end of life, is life the inevitable end of undeath?