Here's some info from the livestream.
The gameplay reveal was the devs cheating. They had infinite health, infinite arrows, could kill enemies almost instantly (wen they shouldn't be killed that easily), etc. Game will be a lot harder. @MaddCow I am sure you like that.
The siege mission the reveal was about is the equivalent of a boss battle. It's not the majority of work, its the result.
Sneaking is viable, mostly during main missions and such, but stealth was upgraded (I don't remember the specifics).
When Deadeye saved the player it was because he was in the area and was looking to help you out. When Ragdug helped the player, it is because he was summoned by the player.
There is open world stuff to an extent. I believe what they said is that each region will have a decently large area to them with missions, people, etc (this is where majority of the game will be), with the forts themselves being part of the open world, not the entire area.
And the last thing I remember, the main character will have a beard. He shaved for the build the trailer was shot in, but as of now he has a beard.
Might have missed some stuff, but the livestream will be posted eventually so that will be here by someone.