I'm kind of burnt on it from playing it a little too much since the latest update, but here's the deal with DF right now:
Vampires can migrate in to your fortress. There's not a ton of indication as to who is a vampire immediately. When the version first came out they were flowing in constantly and would have huge lists of exiles and kills, but now they don't brag about their kills or how many times they've been kicked out of their civilization, so the only indication is unbelievably high social skills and gray hair. Except most vampires know that their hair is a dead giveaway, so they shave it. Usually you won't find a vampire until you start looking for them. And that usually happens after you've lost a few dwarves to it's hunger. To make matters worse, vampires know other vampires and will conspire with each other to make sure you can't root them out. So if there's 3 witnesses to a crime including the criminal, the perpetrator is a vampire and one of the witnesses is a vampire, the two vampires will blame the bystander. Because they're social butterflies and their social skills start high and raise meteorically, if you have a vampire in your fortress he will often end up as the mayor, and if he's freely integrated into your fortress he tends to become a popular dwarf quickly. As a plus, they need no sleep, and neither food nor water, and never get angry at repetitive tasks (just patrol duty) so you can lock them in a 2x1 room and have them pull a lever or turn a crank forever. They don't even need to take breaks. Oh, also: There's a reasonably high chance that, much like demons, Vampires will either be outed very quickly during the early days of worldgen, or integrate themselves into a very high position of authority. Vampire kings, nobles, and bureaucrats are relatively common in every civ and you can have a vampire King or Queen immigrate in with literally tens of thousands of kills. Because a vampire noble never has any reason to flee a society - they just turn it into an oppressive dictatorship as soon as suspicion is raised.
Necromancers can spawn as their own civ, based on a statistic in worldgen called Secrets. Secrets relates directly to books and slabs of power. Slabs are made by powerful demons and deities at worldgen and hidden throughout the world. A person of sufficient age in worldgen has a chance to begin a mad quest for immortality, where they search the world for the secrets of necromancy. If they can find a slab, they'll gain necromantic powers, and as they gain influence they gain apprentices and raise armies of the dead. Once a necromancer at worldgen has created and controls at least 50 corpses, they can create a tower, which operates as it's own civ in the world. This means that when you build a fortress, they appear in the civs tab and can invade. Necromantic invasions are huge, come from all sides of the map, and the necromancers leading the army (usually 2 or 3) instantly resurrect any nearby corpses. This includes corpses in your tombs, graveyards and refuse piles. Needless to say, things can go south incredibly quickly, as undead overwhelm your army and increase in number exponentially. A catsplosion that stumbles into an undead enemy can suddenly turn into dozens of zombie cats, which are more than powerful enough to kill any non-military dwarves they come across pretty much instantly. I have not been able to figure out a good strategy for dealing with enemies from the Tower, and every siege I've faced, no matter how prepared, usually ends when one of my legendary military dwarves gets turned and starts rampaging through the fortress with undead immunities and strength boosts. Did I mention corpses retain all of their skills and can wear equipment? yeah. Oh, one more neat thing: you can become a necromancer in adventurer mode, if you can either find a necromancy book or find a slab. Necromancer adventurers have the same abilities as invading necromancers - they can raise an undead army, and their army has no hard size limit. It seems like necromancy would be the most effective way to wipe out a civ in adventure mode if, for example, you really don't want to deal with humans in the world you generated.
Evil biomes are much more interesting and much, much more difficult now. Once an area is sufficiently savage, corpses can sometimes spontaneously animate and spread undead syndrome to other corpses. I had a shucked mussel shell animate and kill a war dog, which proceeded to kill a military dwarf, who proceeded to wipe out his squad, who ran into the fort and killed everybody. It was horrible. Diseases are a big issue now, but they are only created under specific circumstances, like Megabeasts who can breathe necrotic breath. (needless to say, this is a bad thing) Everything in an evil biome is, well, evil, right down to the vegetation. Eyeballs and tentacles growing everywhere and living trees. Evil weather tends to be things like acid rain, mists that make your dwarves go crazy or animate the dead, or sometimes something as benign as, say, dwarf blood.
On the performance end, DF just keeps getting worse and performance with modern hardware will probably never be good. DF does not support multi-threading or 64 bit processing and probably never will, and as such the best possible setup for the game would be to custom-build a computer with a few gigs of ram and an overclocked pentium 4 processor. As a plus, this would be very cheap to build and could probably serve quite well as a home theater computer, so it's not like it's that big of a deal.
