For the past few expansions, I've enjoyed how Assassination is kind of the "snappiest" of the Rogue specs. In some ways, Assassination is changing the least of the three specs. They're focusing it a lot more on damage-over-time effects - in fact, Assassination will now be the only spec to use both poisons and bleeds (though Subtlety gets a Shadow-DoT finisher that looks a lot like a re-skinned Rupture.)
So let's see what we've got!
Deadly Poison works much as it does, giving your melee attacks a chance to put a Nature-damage DoT on the target, with subsequent applications doing instant damage.
Mutilate costs 55 Energy, attacking with both daggers and granting 2 Combo Points.
Poisoned Knife is a new ranged attack, costing 40 Energy and striking a target within 30 yards, dealing physical damage and applying your lethal and non-lethal poisons, and giving you a Combo Point
Envenom is your major finishing move, costing 35 Energy and consuming your Combo Points to deal Nature damage (based on CPs) and increase your poisons' chance to apply by 30% for a second for each CP consumed plus 1.
Crimson Vial is actually an ability all Rogues will have - an instant self-heal costing 30 Energy on a 30-second cooldown and healing you for 30% of your health.
Rupture is a finisher that does bleed damage over time, lasting longer per combo point.
Venomous Wounds gives you 10 Energy each time you deal Bleed damage to a target that you've poisoned.
Seal Fate gives you an extra Combo Point when a CP-generating ability of yours crits.
Garrote costs 45 Energy and has a 15 second cooldown, dealing bleed damage over 18 seconds and silencing the target if you use it from Stealth.
Fan of Knives costs 35 Energy, dealing physical damage to enemies within 10 yards, with the normal chance to apply poisons. It awards 1 Combo Point.
Vendetta is a 2-minute cooldown, marking a target for 20 seconds, increasing the damage you deal to that target by 30% and allowing you to keep seeing them through stealth and invisibility.
Mastery: Potent Poisons increases your poison damage.
So it's clear that Assassination is going to be more DoT-focused, particularly with bleeds. This should make the revamped Venomous Wounds even more potent. I suspect Poisoned Knife won't really be part of the single-target rotation, but it will let you get some damage on stray targets with ease. So here's how I see the rotation working:
1. Maintain Rupture.
2. Envenom if Rupture's not going to fall off before you can get back to 5 CPs.
1. Maintain Garrote
You'll be opening your fight with Garrote regardless of whether you're in stealth or not, and it will now be a continual part of the rotation, essentially taking the place of Dispatch as your secondary CP-generator. In AoE situations, you'll definitely want to get Garrote and Rupture ticking on multiple targets, though the Energy gains from Venomous Wounds will probably get overwhelming with only a couple targets. I'd guess that at 4 targets or even just 3, you'll start using Fan of Knives as your CP generator. Envenom will still be useful in AoE as it will increase your poison application rate (something Crimson Tempest, which seems to be going away, did for us in Warlords.)
Note: there's a new Alpha build coming out, and so we might see some of these Breakdowns getting outdated soon. I'd still expect most specs to have the same shape, more or less, but a few might get some radical redesigns, and certainly some numbers are going to change.