1 ) Total movement re-write. Previously your movement speed and your combat attack speed were a single value. This made fighting someone while on the ground an extremely one-sided battle. This is being changed in the next release which leads me to:
2 ) Total combat re-write. Now you will be capable of telling your character to actively attempt to parry attacks, along with being capable of attacking with multiple weapons at once. Non-lethal combat now exists, though prisoner mechanics haven't seemed to have changed. You may now also sheath your weapons.
3 ) Total stealth re-write. Gone are the days that stealth was walking in an open field with reckless abandon and a speed penalty. Enemies will now have cones of vision and stealth is based on how quiet you are being which is intrinsically tied to how fast you are moving. I would imagine the ambushing skill makes it much easier to walk/run quietly but I do not know.
4 ) Jumping and Climbing. You can now climb non-smooth surfaces, as well as jump several spaces. This includes horrible failures that result in every bone in your body being broken as I'm sure you can imagine.
5 ) Activated World: Worldgen will now continue as you play, the world will interact with itself and you in a procedural manner as you progress.
6 ) AI controlled settlements: All site types are now returned to the game and then some. Goblin Towers, Elf Retreats, and Dwarf Fortresses will all be represented in-game and created by it just as they were in worldgen. They will be filled with entity populations, historical figures, guards, and all other sorts of fun things.
7 ) Small Scale Warfare: Sites may be claimed by various individuals including you. You are capable of over-throwing the current rulers of a settlement by organizing an insurrection. While Army-To-Army doesn't seem to be a prerogative for this release there's still quite a bit of work going into establishing the groundwork.
8 ) Conversation System Re-Write: Conversations now take time and can be interrupted! People will "remember" the conversations so if you plan to overthrow the local government you might get tattled on.
9 ) Fort retirement: You may now retire your fort without abandoning it. The Worldgen process will take control from that point. You may freely go on adventures and take control back at any time you like. Settle your adventurer in your fort and have him as a citizen if you please. Probably a good and safe way to train adventurers in the safety and comfort of a dwarf fortress.
That's probably not everything but that's a lot of it. A lot of this information was in the DF Talk segments or the Dev Blog. Linking every individual one would be extremely time consuming and I hope you'll forgive me for not doing so.