All the Death Knight specs are getting some changes, and Death Knights as a whole are getting the Rune system revamped in a way that's arguably more profound than the Cataclysm changes (if you started playing post-4.0, during Wrath all Runes regenerated immediately after using them, making gameplay a lot tighter. Cataclysm introduced Runic Empowerment and Runic Corruption to compensate for the slower runes.)
Unholy retains its two major themes - raising the dead and inflicting diseases. This latter one is getting a bit more emphasis than it has in the past, and the former is getting simplified in some ways while getting more options in talents and such.
Unholy will still be the DK spec that does DPS with a 2-handed weapon and has a ghoul pet hanging out with them.
As with all DK specs, you'll now only have a single kind of Rune that can be used for any ability that costs Runes. There's also only one disease to maintain, though Unholy has other effects that play on that theme of infection and decay.
Death Coil remains your main Runic Power consumer, costing 30 RP to deal Shadow damage at range. The healing component for friendly undead targets seems to have been removed.
Presences are gone, but you'll get a passive called Death's Advance, which increases your movement speed by 15%.
Raise Dead still has a 1 minute cooldown and summons your permanent Ghoul pet. (You can still only have one Ghoul at a time, though there's a talent that gives you a skeleton pet in addition to the Ghoul.)
Festering Strike costs 2 Runes and does a large amount of physical damage as well as putting 1-3 Festering Wounds on the target, which interact with Scourge Strike.
Scourge Strike costs 1 Rune, dealing physical damage as well as an additional 50% Shadow damage. It also consumes a single stack of Festering Wound on the target, causing the lesion to burst, thus dealing Shadow damage to the target and generating 3 Runic Power.
Death and Decay looks largely the same, dealing Shadow damage in an area for 10 seconds on a 30-second cooldown. However, while inside of it, your Scourge Strike will hit all nearby targets.
Sudden Doom remains, giving your auto-attacks a chance to make your next Death Coil free.
Dark Transformation is now a simple 1-minute cooldown, transforming your ghoul into a stronger minion with empowered abilities for 20 seconds, which means this should have a one-third uptime.
Summon Gargoyle is a 3-minute cooldown that summons a gargoyle to bombard the target for 20 seconds.
Outbreak costs 1 Rune, dealing Shadow damage to the target and surrounding them in a miasma for 6 seconds, infecting it and any other enemy standing inside the miasma with Virulent Plague.
Virulent Plague deals Shadow damage every 3 seconds for 21 seconds, erupting when the target dies or with a 30% chance on every tick. When it erupts, it deals Shadow damage, split evenly between all enemies nearby.
So it looks like the rotation's a bit more reactive than simply stacking up empowerment on your ghoul. Before artifact traits and talents, here's what I think we've got:
1. Keep Virulent Plague up on all targets.
2. Death Coil if Sudden Doom procs or if you're at 80 or more Runic Power.
3. If there is a Festering Wound stack, hit Scourge Strike.
4. Festering Strike
5. Death Coil
Death and Decay might also be worth it to use in single-target situations. Also, Dark Transformation is on a short enough cooldown that you should probably use it whenever it's up, even on trash. For AoE situations, you'll want to stack up Festering Wounds on all targets before you hit Scourge Strike, which you should try to line up with Death and Decay (if the enemies last long enough, you might even consider holding DnD until you've got a few Wounds stacked up on a few targets so you can pop them all with a single Scourge Strike.