So there's a lot of threads lately on what it takes to tank. As a long time tester and tank for a number of dungeons I figured I'd dispel some of the myths about tanking as well as give some helpful tips for people for tanking.
Blocking Mechanics(UPDATED DUE TO CHANGE FROM PTS: 4/15/2014) Apparently somewhere along the way they changed blocking mechanics to now take a static amount of stamina regardless of the scenario. Upper levels (VR10 testing this) this amount is 216 per hit.
Increasing your block amount will simply provide damage mitigation. This makes passives like "Sword and Board" 20% mitigation basically turn into a flat damage reduction for no additional cost. Take an example of being hit for 200 damage, where 10% is reduced due to armor down to 180. Normally you'd take 90 damage from blocking, now you'd take 72 with that 20% mitigation on Sword and Board effectively turning any % of block amount into flat % of damage mitigation.
Cost Reduction also becomes incredibly productive when acquired. For example "Fortress" giving 30% reduction in cost is just a simple cost reduction. This flat amount also makes "Blocking" enchant incredibly efficient as it'd mean -35 cost is roughly equivalent to a 16% cost reduction over all to blocking.
Armor Type
Generally speaking, armor type doesn't matter. I tank just fine in Light Armor and there's many bosses in the game that chuck spells at you as much as they hit you hard. Generally speaking there's a large number of armor buffs (Combat Prayer, Lightning Form, Volatile Armor, Circle of Protection, etc) that can fill that gap and still give you the benefits of being in Light Armor (reduced magicka costs and such).
The main advantage to wearing Heavy Armor, now that we understand blocking mechanics, is that Heavy Armor's Bracing 2/2 will reduce Blocking Cost by 20% which stacks with 30% from 1HS which brings Stamina Cost of blocking damage 1:1 which effectively means while blocking your Stamina is a second health pool. A second health pool that regenerates much quicker than Health.
Weapon
It's important to recognize that as a tank, game mechanics are designed in such a way they really only want you tanking 1 thing at a time. There's really no massive AOE tanking and even if there was there's so much damage from that many sources you'll likely die or be stamina starved constantly.
So for "trash" pulls I generally like AOE bars, such as Destruction Staff, Dual Wield or Bow along with AOE class abilities. Most cases here the object is to do as much CC as possible and keep everything together and AOE it down with your group.
For actual tanking, really do want a 1HS given that blocking mechanics are greatly enhanced while wielding a 1HS. Really there just is no alternatives and while you can, certainly, tank in other weapons it's not ever going to be as effective for damage mitigation as a 1HS.
Taunting
There are in fact two taunts in the game.
Puncture - Puncture is rough because as an ability it costs a boatload of stamina which you also need/want for blocking. It's very easy early on while tanking to get caught in a scenario where you block, then have no stamina to taunt. The reduced armor/spell resistance is nifty, but ultimately if you look at the numbers amounts to 8-10 damage extra or so which is pretty non factor. Early on this is pretty much the only taunt available however so if you don't see a tank without a 1HS chances are they don't have the ability to taunt (lookout healer!)
Inner Fire - This taunt is available in the Undaunted line and requires 13 achievements from the Dungeon Line to unlock. This means completing 13 dungeons generally speaking. You typically won't get it before the 40 dungeons unless you really run a lot of dungeons and get extra achievements (Dungeon Damage, Dungeon Ruler, etc). This is a Magicka based taunt and it will change everything game play wise. Now you can use Magicka to taunt which will now allow you to save Stamina to block. It's also a ranged taunt and can be morphed into Inner Beast which gives you 28m range and on top of that also reduces it's cost.
Your Role
Generally speaking I like to do as much AOE CC as possible on trash pulls (in my case I use Talons) and whatever AOE I can fit in. My bar tends to be Destro Staff since I am rocking Light Armor. I try to slip into the back to interrupt healers if I see them and otherwise just keep everything AOE Rooted as much as possible. If there's a big mob (Lurcher, Centurion, etc) then I tend to grab that and let others AOE instead. Trash in this game was meant to be AOE'd and not tanked and you just gotta accept that. By doing AOE CC you prevent the trash getting to your healer which is an acceptable solution.
Against bosses of course I primarily taunt and do my best to stay out of bad areas as well as managing my block. Managing block and when to block and when not to block is pretty important. If you just block the whole time you will have no stamina what so ever. Eventually you learn a rotation of what to eat and what not to eat as well as recognizing when the boss is going to do something bad to you. For example there's really no televised attack on Bogdan in Elden Hollow VR10 when he does his flame breath but not blocking it is around 1500 damage so you gotta learn to just kinda time it out and when he's going to do it and ride block till he does then ease up a bit and regen Stamina eating some hits.
Stamina Management (blocking, dodging, debuffing with Low Slash) is pretty much the entire job.