Some notes from the video:
-Around 6 hours of new missions including an epilogue with about 8 missions. These are meant to focus more on the notable pirates you run into it seems.
-Modern day stuff is confirmed to be removed (I think it was already confirmed but I still see people confused on that). In place of it are little missions/scenes that focus more on Kenway's story.
-No more secondary objectives on missions that give you full sync (a bummer to me personally but not the end of the world). They redid the structure of missions to get rid of anything that made it seem like you were playing as someone experiencing things through the animus, so no more "full sync" objectives and no more pop up screen at the end of missions.
-They redid weather and night time to make it so you can sometimes struggle to see when sailing. This also helps with stealth on land though, rain and night decrease enemy visibility. You can also crouch on command now.
-Entire UI and menu system has been redesigned. Have to check the video for examples, but IMO it's an improvement.
-You can dive underwater anywhere now it's no longer restricted to the diving bell areas. Some locations have been redesigned to incorporate that too. Example in the video: a pirate cave where you have to loot chests to complete it has one of them in the bottom underwater.
-Fort attacks have been tweaked. You no longer finish them by beelining to the commander and taking him out. You have to wear down the enemy forces by killing a number of guys, then the commander.
-He thinks the combat will be the most contentious part of this and the hands on part left him underwhelmed initially until he spoke with the director and got him to explain what their thought process was, how you can technically spam R1 on many enemies but they want you to experiment with the tools he has too and get into a flow (sounds similar to batman combat in that you can mash square but you're gonna miss out on 90% of the combat if you do). The director does say they purposely allowed for a bit more spamming R1 in the northern regions but the southern regions are harder.