I think after having sufficient time to experiment, a lot of things about this version have been figured out. This is a place to share your knowledge and thoughts on the Valentines Day update. Vampires, Werewolves, and the Undead! Let's go!
What we know so far
Fortress Mode:
Is a completely random syndrome fog that sweeps across the land.
Can be harmless, or deadly depending on if you are lucky or not.
For example, it can instantly turn any creature in it's path into a zombie, turn them hostile, and leave behind a pile of creeping ash.
Can go over walls if they do not have a roof.
There can be multiple clouds at one time at any given embark.
It's name is random.
Appears in the form of a miasma-like cloud.
A randomly generated material rain.
Like regular rain, it is unpredictable in when it will come.
The entire embark can be covered in whatever material is generated.
It is fairly common to get blood rain, and have the entire embark be covered in a type of blood.
After a creature being killed by an undead creature, the corpse of the creature will re-animate.
Will always be hostile.
Can be killed over and over and will continue to rise.
If a creature killed by an undead creature is butchered, all of the parts will come alive.
Can siege your fortress like any other race, despite not being a "race" exactly. You get the message "The dead walk. Hide while you still can!"
Some corpses maintain the ability to fly after death.
Possible ways to kill include magma, atom smashing, and cutting into tiny bits. (Last method requires verification)
Gain the NOT_LIVING and OPPOSED_TO_LIFE tags.
They suck blood and are disguised as a regular dwarf upon arriving.
They feed on sleeping dwarves.
If another dwarf spots them feeding, they will accuse them of being a vampire.
In adventure mode they are often the objective of quests, but it is very rare to encounter one in beat form.
On a full moon, they will attack your fortress, similar to a siege. (Verification needed)
While in human form, werewolves do not get any of the benefits of being a werewolf.
Other Werecreatures exist, the creature they turn into appears to be random.
If they attack a dwarf and the dwarf survives, they become whatever were-creature that attacked them.
Adventure Mode:
Will awaken if you move to an adjacent tile, followed by awakening all nearby skeletons. This can be prevented by moving all the bones of skeletons away from the area before alerting the mummy, making the number of undead you face significantly lower. (Verification needed)
Re-animate the dead.
Necromancers show up with neither a birth nor death entry in the legends.
They take on apprentices.
They create factions.
Live in towers marked by a purple I on the map (with vanilla graphics).
Undead are not hostile to these units.
They do not age.
Can appear at a fortress. While at your fortress, can light campfires.
In the legends, they learn their art by worshiping a deity.
Will raise their own severed limbs.
Awake one turn after any nearby mummies do. (Verification needed)
The Undead seem to die quickly from blunt attacks.
On certain creatures different weapon types are more effective than others.
Miscellaneous/Interface changes/New Things
Thirst and hunger were re-added.
The units list is now divided into four tabs. Citizens, Pets/Livestock, Others (Wild animals) and Dead/Missing.
If a unit drowns inside of a pond, it will count as missing until another unit spots it.
Cannot stop a job once it has begun (i.e. digging, chopping down a tree) (Needs verification)
Upon reclaiming in a terrifying embark, the message 'Ambush! Drive them out!' appears and the dwarves of the previous fortress attack.
Staircases can now be designated across z-levels.
Any contributions are appreciated (unless it is something to the tune of "Vampires bad, magma burn hur hur"). Some of the things in here are unverified and should not always be taken as fact. If you find anything to be false, please let me know and I will fix it accordingly. Also, expanding on things already here is encouraged.