Here's something I've noticed with my own tank, and from talking to friends who have tanks.
Healers get better gear, their heals get better, they have more mana, it's peachy.
DPSers get better gear, they do more damage, they have more health, more mana, et cetera ... peachy.
Both roles can be fulfilled in a high-level instances or a slightly lower one. You can go to Karazhan and then go to Mana Tombs for a friend.
But for tanks? You get better gear. You take less damage, you have way more health, you can take on tougher and tougher things. But you go into a lower level instance in that gear and you can't tank for toffee. Not because you're getting hit, but because you AREN'T getting hit. So in my case, I don't generate rage, so I can't build any threat. I'm even finding it happens in level 70 instances on normal now, especially on trash.
It's a problem. I'm finding that I have to wear some of my crappy DPS backup set to be able to generate any rage. I know that a paladin friend of mine has to do the same to be able to avoid mana starvation.
Solution? I'm not sure. Maybe make it a protection-type talent that not only do you generate rage/mana when you get hit, but you also generate it when you dodge, parry or block. That's about all I can think of.
Done the same here, it's silly that better gear = less threat, but there it goes. I tanked BM once dual-wielding on the Rift Lords 'cause I was otherwise just doing no threat, with the duals I was able to spam a lot of Heroics. Still had over 12k armour.
The other way to increase your threat is to stack Hit and Expertise - they're a lot more common on top end gear for that very reason, that since most tanks have been effectively been rage capped since Gruul (if something's hitting you for 7k every 2 seconds you'll have all the rage you can want) the only way then to get more threat is to land more threat causing abilities in the same time. Having zero hit on talents and only 6 Expertise means you can get a LOT and still not be capped.
Consider this: The base miss rate for a L70 vs a Boss level MOB while single-wielding is 9%. A Boss' base chance to Dodge or Parry (always 15 Def more than your max weapon skill) is 5.6% for each. So you only in fact have a 79.8% chance to get a Hit result on any of your attacks, including Heroic Strikes, Sunders etc. Now each point of Expertise reduces enemy Dodge and Parry by 0.25%, so you need a total of 23 Expertise to completely remove Dodge and Parry results from the hit table.
In total this works out that you need 142 Hit rating and 90 Expertise Rating to ALWAYS hit with all your attacks, including things like Sunder, Revenge, Heroic Strike etc. This should thus equate to a 25.3% increase in Threat generated.
Considering you're a Prot Squidface, you've already got at least 1% Hit and 6 Expertise from Racial and Talents, which is bloody helpful.
The other thing to consider for max threat is Shield Block Value. Not Rating, which is fucking pointless for Warriors since with talents, base values etc, you will guaranteed Dodge, Parry or Block every attack vs a single target while Shield Block is up (unless they have silly attack speeds, but then Block Value and Dodge are better anyway). Block Value makes your Shield Slams hit harder (and with a Fury/Prot PVP build you can slam people for like 7k crit) with the threat from Shield Slam scaling with it's damage.
So yeah, like all good Warriors you should look to building basically two sets of gear - one geared for Threat and one for Survival. Threat should have priority for Hit, Expertise, Shield Block Value and Stamina, while Survival you want to Prioritise Dodge, Parry, Armour, Shield Block Value and Stamina. Oddly enough Defence is one of those things that's only worth looking at if an item has LARGE values of it as that adds to all three avoidance stats, otherwise just hit L70, sneeze and be at 490 and Uncrittable.
As you might have guessed, I can talk about Warrior theorycraft forever, what with us basically being the single best, most interesting and most involved class to play in the game.
You cringe at the guild chat saying "please come tank SV so and so really needs there key frag for Kara"
Most likely you'll be in there with some overpowered rogue/mage/lock that has already landed 5 crits before the trash even has a sunder.
So I say let the mage tank it and just sneak your way to the boss, that's all they really want you there for anyway.