So, big caveat here: my Death Knight was a Blood Tank through Wrath and Cataclysm and then went Frost in Mists and Warlords. I've never spent a ton of time playing Unholy, so there are, I'm certain, some nuances to the spec that I'm less familiar with.
That being said, of the three DK specs, Unholy is probably changing the most (well, maybe it's on par with Blood) and so we're kind of in a brave new world anyway. Unholy is still going to be built around a theme of disease and you'll still have your permanent ghoul pet (though this guy can be talented to become an abomination, which is really cool though I think it's high time the standard abominations get a model update akin to what the various old-school warlock demons have gotten - not a new design, just an update.)
However, there are a lot of new mechanics going on, so let's cover them:
Remember the Runes:
Most importantly for all Death Knights is the simplification of the Rune system. All Runes are now a single type, and so you'll never have to worry about wasting a Death Rune on the wrong ability - effectively all Runes are Death Runes now. You'll still only be able to regenerate three runes at a time, but they're not paired up like they were when there were three different rune types, so just the thee runes used longest ago will be regenerating while the more recently used ones will be queued up.
Each DK spec now has a single disease. With Blood getting Blood Plague and Frost getting Frost Fever, Unholy gets a new disease called Virulent Plague. This is applied by the new version of Outbreak, which costs one rune (no cooldown) and applies a miasma around the target that infects it and all enemies nearby with Virulent Plague. When a target dies while afflicted by VP, or just as a 30% chance when it ticks, the target will erupt with a burst of AoE Shadow Damage that is divided among targets nearby. This should provide some pretty solid cleave damage (yes, the burst is divided between targets, but you also very easily infect all the targets in a pack.)
With diseases far easier to maintain, the purpose of Festering Strike, as well as Scourge Strike, has shifted. These two attacks are the core of your rotation.
Festering Strike still costs two runes, and it deals pretty heavy physical damage to the target while also applying 2-4 Festering Wounds on the target. The number is random, and is something you'll have to pay attention to. On their own, the Wounds don't do anything. But that's where Scourge Strike comes in.
Scourge Strike hits the target for physical and shadow damage, and then it consumes one Festering Wound, which pops, dealing additional shadow damage to the target and generating a bit of Runic Power for you. If you're standing within Death and Decay, Scourge Strike will hit every enemy near the target as well.
So I could actually imagine a cleave/AoE rotation that involves hitting multiple targets with Festering Strike, then putting down D&D and hitting Scourge Strike to pop them all. I think that's the intention.
Less Ghoul Management:
With the new focus on Festering Wounds, you'll have a bit less to worry about in preparing for or maximizing Dark Transformation. The ability is now a simple 1-minute cooldown that has no prerequisite to cast, though you can pick up a talent that causes Death Coil to cut seconds out of its cooldown.
Army of the Dead:
This ability is now Unholy only. So, grats.