PvP'ing as a Tank

Arguably the hardest thing to do in WoW, attempting to PvP as a Protection specced character, the classic sword and board style, is an exercise in incredible frustration.
The hugest problems you have is the fact that your damage output is... well, embarrassingly low. You'll have all these people running around with 200 resilience, and you'll be like "Why bother? I have, like, 11% crit. I couldn't critically strike a critter if I tried"

The problem is simple, really. You cannot think to yourself "Ok, how the hell am I going to kill this guy?".

Ask yourself. Do healers see a hunter running up to them and think, "Holy crap, how am I going to kill this guy?" No, no they don't. They immediately look around for someone else who CAN kill the hunter, and then help them do it.

Quite bluntly, you cannot think only with yourself. You have to think about how you can interact with those around you to be successful.
Go read this post about tunnel vision.

The biggest key about being successful in PvP is realizing that you are not alone. There are other people around you, you don't have to kill that evil mage all by yourself. In fact, you most definitely couldn't. You have to start thinking with those around you.

If you are a mage in PvP, what do you do? You crowd control, you deal damage, and you stay at range. Thats what we're good at, thats what we do.

If you are a Prot Paladin, what do you do? You get up close, try to get aggro, and give the DPS people as easy a time as possible to kill the badguys.

Of course, PvP =/= PvE, but some principles hold the same. There is still such a thing as aggro, for example.
Aggro, essentially, means somebody wants to kill you. If you charge somebody and start hitting them in the face with an axe, they will want to kill you.
If a Paladin starts healing you, if the person your hitting is smart, he will switch to trying to kill the Paladin. The Paladin can wipe his aggro by bubbling, therefore rendering him invincible. Its temporary, but the enemy player will no longer be able to kill him. Hence, aggro wipe.

If you are a prot spec warrior, you cannot simply whip out a massive 2H mace and start hammering people. You aren't going to be good at it, thats not your role.
Same way as a rogue cannot suddenly switch to a ranged DPS role, or a warlock to thinking she can go around melee'ing people.

I mean, you can, but you'll fail. Quite miserably.

Let's take a completely hypothetical example.
Say you are standing at Blacksmith, there is a mage on your left, and a holy paladin on your right.
The mage is currently sitting down, and drinking.

Suddenly, you see a hunter come around the corner of the building, and target the Paladin. You are not in combat.

What do you do?

This should be easy. Your healer is in danger! Help him!

Charge the hunter, grats, you stunned him. Now the hunter, if he wants to kill anything, needs to get you out of the picture.
Act like a standard warrior, slap him with hamstring as soon as possible (the pet too, if you can), and start laying the smackdown, as best as you can. Whip out dual weilding swords if you like, and unleash a Devastate.
Suddenly your Frozen! Oh noes! Trapped! You trinket out, and then...
LOOK AROUND. NO TUNNEL VISION, REMEMBER?

Remember that mage who was drinking? Guess what? He just got Ambushed. Move yer ass over there and get in that rogue's grill!

Intervene, whatever, save the squishy!

Keep in mind that, if the mage is thinking like you, he will see that hunter out there, you on the rogue, and get around to sheeping the hunter at some point. If the mage isn't that smart, well... you did your job.

Ideally, you will tag the rogue with Rend and Hamstring as soon as you can. Keep that rogue stuck next to you, and a DoT ticking so he can't Vanish or anything like that. Keep that bleed effect up, and some of the rogues best tricks will be gone.
If the mage is smart, he's already running from the rogue, running through his own head what he can do here.
Loose hunter? Off goes a polymorph. Whirling around, he immediately starts hitting the rogue with everything he's got. Oh noes! You got blinded by something, and the rogue shadowstepped right behind the mage.
Trinket out, intercept to the rogue, keep on him. Give the mage as much breathing room as possible to unleash some more painful hell on the rogue.

The key is to be as ANNOYING as possible. Rogue pop sprint? Hit him with Concussive Blow, whatever it takes to keep him as pissed off at you as possible. Stay on the rogue until the mage can take over, like when the rogue reachs 30%, is rooted, and the mage is charging up a Shatter combo. Keep Rend up to prevent any last ditch Vanishes.

Same thing versus the hunter, when you have the opportunity.
Concussive Blow, devastate while dual weilding, hell, even stack up some sunders to piss him off.

Whether the made up WoW players did the right thing or not, doesn't really matter. The fact is, YOU are a TANK, so act like one.
You are NOT a mortal strike warrior.
You have a shield. You have Vitality for goodness sake. PLAY like a tank.

See a rogue suddenly leap out at your healer? Intervene! Intercept! Lay some quick movement impairing smackdown to give your healer time to get away.

See an enemy mage near you starting to cast sheep at your healer? Shield bash him!

And if people start hitting you with stuff, thats exactly what you want. You can take the damage, the Holy Priest behind you can't.

For Paladins, its exactly the same. Except replace Concussive Blow with Hammer of Justice, and Rend with Consecrate.

The hugest problem with this is that you will, in essence, depend a great deal on the people around you. And sometimes, they will not come to your rescue.

Sometimes, you will make the perfect setup for escape.
Like that mage earlier. You intervene, slap the rogue with hamstring + rend, and the mage promptly Ice Block. Great. Just great.
Even better is when the mage leaves the hunter unchecked, and the hunter goes off and starts trying to kill your Paladin.
Frustrating? Damn straight it is.
Yo gave that mage the perfect set up to get to range and unleash some delicious damage without having to blow any of his cooldowns, and he goes and immobilizes himself right next to the rogue.

And that happens, it really does.

But again, you really have no choice. You are a tank. You have to depend on those around you to do the damage for you, to heal you, to keep you up, and it is your job to protect them as best as you can.

A hunter trying to flee from a warrior? Hit the warrior with Avenger's Shield. Slow the brute down.
Intercept him. Throw down some Sunders, so the hunter can do more damage. Nail him with Hamstring, so the hunter can easily run away.

Warlock trying to fear your healer? Shield bash it, hamstring both the 'lock AND the 'locks minion, especially if its a felpuppy. Keep those evil things off your healer.

Keep this in mind. You have a specific role, according to your spec. You should know what it is, and play the spec. Do the best you can with the resources you have available, and try to work with the people/classes around you.
Some of them will know what they're doing. Some of them will recognize a perfect hamstring combo as the perfect opportunity to get the hell out of there.

Quite bluntly, you are a tank. Act like it.