From Kotaku:
The game's new co-op missions are optional, all story-based, but can be played solo, too. You find them in the game world. The developers promise that those of us who love playing through the game's solo campaign and side missions will not feel underserved.
There is no competitive multiplayer mode in Unity, the first time a big new AC will lack that since 2010's Assassin's Creed: Brotherhood.
There's a crouch button. Yay!
There will be a modern section to the game as well, in some way starring "you." I'm not quite sure what that means, but they said it'd be different than Assassin's Creed IV, which had the player in the role of an unnamed employee of fictional (and Templar-aligned) Abstergo Entertainment.
Amancio may have previously been the creative director of Assassin's Creed Revelations, but he promised me we would not be getting the return of that game's tower defense missions. Praise the lord.