biggest change is the awakening of the world. Succession worlds can happen now as new events, like births, keep on going even after world gen. This also means bandits may decide to camp near your fort halfway through the first year, or a necromancer can set up a tower while you're playing and visit after a few years.
sieges were drawn partly from historical figures already and killing generals could lead the attacking civ to lose the ability to use some trained monsters
IIrc, settlements other than human are also present in adventure mode now.
the combat system recieved a complete overhaul
Morale... recruits run away because there are so many scary goblin corpses. Quite realistic ofcourse, but slightly problematic. "Dwarven childcare" almost becomes mandatory.
Traps have been sligtly nerfed as far as I understood. Also, due to climbing, just putting a wall around your fort isn't going to be enough.
Quite a lot of small bugs have been fixed, read the devlog (
http://www.bay12games.com/dwarves/) and some havn't. marksdwarfs and bolts are still problematic