How to Tank in Cataclysm ~

Important questions -
Times, they are a changing. Go look in the mirror and ask yourself, do I really want to tank? Seriously, go ask or at least turn your monitor off and look into it. Now, let me ask, are you ready for the coming changes? This might sound crass but looking at the beta vids, hearing Blizz talk and the fact the changes we have now in 4.0.3a tanking will be different.

Oh, I have a bit of a gift for you all, more on that later.

Back to the future -

*This is something we must all make the adjustment with but if you have only tanked in cataclysm please re-read this.

I am not going to sugar coat it this should be a guide to point us that way. Wrath was easy on us all, if we make adjustments, play smarter and play with in our toons limits it will make the runs easier for all involved.

Back to the basics -

Mark your adds - Allow the DPS to see what you need dead first. Tell them to target your target with cute little icon over the NPC's head.

Use CC - You have it, use it. This will help your group as well. All accounts Cata is going to be harder on us all, healers will have less mana to play with and less damage makes their life easier.

Pull smarter - Learn to LOS, let your group know you are doing it. Use the body pull as well. If CC goes off and you throw shield or holy wrath you could break CC and we don't want to do that. It’s why we used CC. This body pull thing... learn how to do it, its a controlled face pull.

Give instruction - Tell the DK, Hunter, Mage you need them to kite. Let the lock know you need seduce and make sure no one is to far bomb the group.

Take suggestions - Regardless of our queue times, we are not the royalty of the group that we think. Yes, Cata DPS will not be able to do a heroic without you but it doesn't mean you need to be an asshat. You are there to take damage and reduce damage to others, let them know you know what you are doing and if someone says "hey I seen it work this way" do not rage quite or tell the group, kick me or GTFO. Your heroic wrath gear will not say what a big shot you are any longer.

Improvement -

Know your toon - Put a bit of time into learning your toons. What do I mean by this? Know cool downs, heals and damage reduction. Be liberal on your Cool downs and use stuns. Is this a caster group? Have Resistance aura up instead of Devo. You have hands for a reason, know when to put a hand on someone in your group.

Know your enemy - Sure there will be an adjustment period, a time when no one in the group can find their behind with both hands. But have an idea of what you are facing. Expect on pull a burst of damage. Know when a special ability is coming and let the group know. In other words, things will be dangerous never underestimate your opponent.

Assist the group - This goes back to healer’s mana and you soaking damage, oh and using tools. If there is a dot on you; cleanse it. If the healer has one on him, cleanse it. LoH, Salv and BoP all work wonders at helping the group. Let them know you know how to tank. I believe that a well-placed hand or LoH's sends the message, you are the go to guy/girl in this place.

Don't be "that tank" - Know what classes have what CC, what buffs and by all means allow the healer to have mana. Don't ask a mage to sheep undead, it won't work. You will look like a fool!

Use buffs - This is no longer Wrath; you cannot zone in and pull. At least initially, use some consumables. Pots, food and buffs will help you along. There was a time when a tank was not in any way shape or form part of DPS that has changed. Do what you can to speed the run by taking a second to use your toons potential.

TL;DR - Use common sense as the tank; mark your adds, use your tools and don't be a tool. But I suggest you read this, its not really that long.

There is a great post on MMO champion along these lines. Good read and well, its MMO Champ so you know the responses you get but the OP gave a more detailed all role break down.
 
My Gift -
 
Some might know this, lots don't but here is my gift to you as a reader...Clear your quest log out, everything, clear it out. Go grab 25 dailies and do them until December 7th. You should by now have your hearth set to your home city. As Wrath goes live, spend the first 10 to 15 minutes turning in your 25 dailies and you now have 1/3 to 1/2 of a level and some extra gold...FREE! Hearth to your home city and start your journey to level 85!!!