Well, in this long series of "breakdown" posts for the various specs (and yes, I realize that I haven't done all of them yet, especially healing ones,) I made a (if I say so myself) valiant attempt at figuring out Vengeance, but there wasn't really enough there to put it all together.
There's a new Alpha build out now, and while the biggest difference I can detect is that there seem to be a lot of different spell icons, I think we're starting to get a real sense of what the spec is supposed to play like.
First off, it seems that the spec is going to use a different primary resource than Havoc. So in place of Fury, you'll instead have Pain, thus making Vengeance the most emo spec in the game! I don't know if Pain will automatically generate from damage taken (which would make a lot of sense,) though I think if it did that it would be dangerously close to Rage. If not, I think you'll just generate it a lot slower than you do Fury in Havoc spec.
It looks like parrying, healing, and straight damage reduction are big themes for the spec. Vengeance will wind up being probably the most mobile tank spec as well (though Warriors might remain close.) So let's get into the abilities. Some of these abilities might actually be for Havoc - there's still two versions of Toss Glaive, for example, so I'm going to try to keep this to things that look like core rotational abilities.
Demon Spikes is an active mitigation ability. It costs 100 Pain (which I assume is the full amount) and has two charges on a 15-second recharge. It increases your parry chance by 20% and reduces the damage you take by 8% (though I don't know if this is before or after baseline mastery.) It lasts 6 seconds.
Demonic Wards gives you your crit immunity and boss-hitting expertise. It also reduces magical damage taken by 30% and increases your stamina by 20%
Fiery Brand has a 30-yard range and a 45-second cooldown. It deals fire damage to the target and reduces the damage they do to you by 40% for 10 seconds.
Immolation Aura has a 10 second cooldown, dealing a burst of fire damage to enemies around you and then continues to damage them over 6 seconds.
Infernal Strike has a 30-yard range and 2 charges on a 15-second recharge. You leap to a targeted location and deal fire damage to enemies within 6 yards.
Mastery: Fel Blood reduces the damage you take while Demon Spikes is up, and it also increases attack power (like all tank masteries.)
Metamorphosis has a 3-minute cooldown, transforming you into a demon that has 30% more health and takes 30% less damage for 10 seconds. While it's up, you also automatically have Immolation Aura up.
Shattered Souls is the same as for Havoc, causing enemies you kill that yield experience/honor to drop a soul fragment nearby. Running over it will heal you, and if it's a demon that you killed, you'll also get a damage buff for a few seconds.
Shear is a melee strike with a 3-second cooldown, dealing moderate damage and spawning a soul fragment. It also generates 5 Pain.
Sigils are a few different spells. You place a sigil on the ground that activates after a couple seconds, affecting targets standing inside. Sigil of Flames deals fire damage over time. Sigil of Chains snares targets, and Sigil of Silence silences them. They each have a 1-minute cooldown (not sure if it's shared) and a 30-yard range.
Soul Cleave costs 400 Pain (guess it might go up to 1000?) and deals high damage to all targets in front of you and consumes all soul fragments nearby, each healing you for 2%, as well as an additional 10% of your max health regardless of soul fragments.
Throw Glaive has a 30-yard range and no cooldown, dealing light damage to a target and bouncing to up to two additional targets.
Felblade is a melee attack with a 10.5 second cooldown. You deal major fire damage to the target and generate 20 Pain.
So here's what I'm seeing: you have Demon Spikes as the cheaper active mitigation option - something you'll probably want to maintain as much uptime on as possible. Fiery Brand works a bit as a quick cooldown - obviously most effective against single powerful attackers. So let's talk rotation:
1. Use Soul Cleave if your health is low - you may want to save this for emergencies, stacking up Pain and Soul Fragments for it.
2. Demon Spikes will probably be your default AM ability.
To be fair, we don't really know how Pain is going to work. It seems unlikely that you'll only get it through abilities, since there are only two that generate it and the cost of the spenders is so high. It might be that you'll mostly get it through taking damage or something akin to that.
1. Felblade on cooldown.
2. Shear (every other GCD.)
3. Immolation Aura - possibly worth it even if you're in single target situations.
4. Throw Glaive - if you don't have anything better to do - though this will probably be mainly for pulling - sort of like how Avenger's Shield used to be before Cataclysm.
Sigil of Flame coupled with Immolation Aura and Infernal Strike should make it a lot easier to pick up packs of adds. The cooldown in addition to the resource cost for Demon Spikes means you might actually find yourself using Soul Cleave more frequently than I'd predicted, but it really remains to be seen how quickly you can generate Pain. Clearly, it's got to be enough for them to justify a cooldown on Demon Spikes.
While I'm not as confident in this analysis of the spec as I've been with existing ones, I do think that we've now got a real idea of how Demon Hunters are going to be expected to tank.