What Ange said, basically
Peterson has a frost tank spec and an unholy tank spec, which is generally good for fights with a lot of magic-based rather than physical damage, and unholy is also very good for AOE threat, with the reduced cooldown on death and decay, the 3rd disease and the extra DoT from unholy blight. Frost has howling blast which is also good AOE threat.
You should be aiming for 540 defense (if you're using a 2H weapon, then you can get 25 defense from the Rune of the Stoneskin Gargoyle, reducing the amount you need on your gear) and around 26k health unbuffed for raids (around 22-23k and 535 defense for heroics).
After that, it's a matter of personal preference in how you stack your stats. A lot of your sustained threat on single targets will come from Rune Strike, and this means you need a decent amount of runic power generation, and a healthy amount of mitigation.
As you gear up, it's nice to build your hit rating and expertise, as missing with a 2H weapon can mean a nasty drop in threat when you've got a rampant aggro monkey like Woogle pyro'ing away behind you. Otherwise, keep your dodge and parry ratings balanced, and if you can match up socket bonuses using red or blue gems, then strength and stamina are always good to add for threat and survivability.
