It's odd, I thought I had done this already, but maybe I just started a draft that I never posted. Anyway!
Guardian, despite not technically existing until Mists, is actually one of the original tank specs (back in my day, we called 'em Feral Tanks!) Way, way back, during Burning Crusade, tanks who wanted to raid needed to get geared enough that they wouldn't be killed by "Crushing Blows." Warriors could easily do this by getting enough Defense Rating to be crit-immune, and then just use Shield Block to easily cover their avoidance table. Paladins had to work harder, since Holy Shield only gave them 35% extra block instead of 50% like Shield Block used to. Druids, whose leather gear never had any Defense Rating on it, could never do this. So what did they do? They'd stack stamina and armor to the high heavens and just soak those nasty blows.
Over the years, this designs has kind of faded away, and instead, Bear tanks have become extremely dodge-focused, using their natural agility (and their Agility, found on all their gear) to become very hard to hit - despite being a giant mass of shape-shifted fur, muscle and bone.
Well, things are changing! Guardian is getting a pretty nice and welcome redesign to reiterate the old design - Bears are tough. Bear tanks are massive beasts, and hitting them just makes them angrier.
As always, this is based on an Alpha build, so expect things to change. But here's what we have so far:
Bear Form is obviously what you'll be in most of the time as Guardian. It increases armor by 250% and reduces damage taken by 20% (there's another passive called Thick Skin that bumps this up to 30%.) It reduces your chance to be critically hit or parried by 10%, and also makes you immune to polymorph effects. Shifting forms will free you from movement-impairing effects. Being in Bear Form also allows you to use your Bear abilities.
Mangle has a 6-second cool down, generating 5 Rage, slowing the target by 50% for 12 seconds, and dealing a huge amount of damage, plus an additional 20% against bleeding targets.
Moonfire is an instant-cast spell, doing a bit of Arcane damage up-front and then a lot more over 12 seconds. It is now usable in Bear Form, which I think is really cool, as I often feel like the animal-druids kind of miss out on the magical side of the class. It's another way to distinguish Guardians from Warriors.
Maul costs 20 Rage and has a 3-second cool down, dealing a fair amount of physical damage to the target.
Thrash has no cool down or cost, simply hitting all nearby enemies for a bit of physical damage up-front and putting a bleed on them.
Lacerate has a 3-second cooldwon, dealing a fair amount of physical damage up front and then making the target bleed for a fair amount over 15 seconds, with the bleed stacking up to 3 times. Lacerate has a 25% chance to reset the cool down on Mangle.
Barkskin has a 1-minute cool down, reducing all damage by 20% for 12 seconds and can be used while incapacitated or otherwise cc'd.
Survival Instinct is your major damage-reduction cool down, with 2 charges on a 3-minute recharge, reducing damage by 50% for 6 seconds.
Ironfur is one of your main active mitigation abilities (though recall that in Legion, cool downs like Survival Instincts also count for any attacks that specifically look for active mitigation.) It costs 40 Rage and has a 1-second cool down, increasing your Armor by 100% for 6 seconds. Subsequent applications of this can overlap, which I suppose means you can actually have many times your normal amount of armor if you have the Rage.
Frenzied Regeneration has a 2 charges on a 20-second recharge and costs 10 Rage. It heals you for 100% of the damage you've taken over the last 6 seconds over 6 seconds, with a minimum of 5% of your total health.
Mark of Ursol costs 40 Rage and reduces magic damage taken by 30% for 6 seconds. This also counts as active mitigation.
Mastery: Nature's Guardian increases your maximum health and your healing received, and like all tanks, it also increases your attack power.
So the picture's relatively clear: Ironfur in particular is going to make you very tough. Essentially you're going to have a ton of health and armor, but you also get a boost to the healing you receive so you don't just become a mana sponge. Technically, the "higher health and healing received" effectively amounts to "you take less damage," but I doubt the healers are going to complain as their HPS gets inflated.
So here's what the rotation looks like, before we get into talents or artifact traits:
1. Frenzied Regeneration if you just took a big hit or big series of hits.
2. Mark of Ursol if there's a big magic attack coming.
3. Ironfur most of the time, and possibly pooling 80 Rage if you know there's a really big hit coming so that you can get two stacks of it (assuming that's how it works.)
1. Mangle if Lacerate procs it.
2. Lacerate if there's fewer than 3 stacks on the target or it looks like it's going to fall off.
3. Maintain Moonfire DoT.
3. Maintain Thrash.
5. Maul, but only if you don't need the Rage for active mitigation.
Depending on how much Rage you get from normal combat, you might want to just use Mangle on cool down. In AoE situations, Thrash is going to help you pick up and hold lots of adds, but if you feel like getting fancy, you can multi-dot Moonfire and Lacerate. The latter might even help with Rage, depending on whether the bleed from Lacerate can proc Mangle. In terms of defense, I think Ironfur will be your AM-of-choice 9 times out of 10, as Frenzied Regeneration is more of a reaction to bad stuff happening, and Mark of Ursol is situational.