The following info comes from Ubisoft's Ashraf Ismail, Game Director.

The main game will be 15 – 20 hours, “but the goal is to have you get lost in the world, spending even more time in it!”

“Edward’s four flintlocks actually reload faster than Connor’s… It’s that way so you don’t have to wait around forever.”

“[Assassin's Creed] is about credibility and authenticity, not about simulation.”
Answering the question of who replaces Desmond, Ashraf said, “You will, you replace Desmond. Playing in first-person.

Black Flag will have some “Assassin tombs like in [Assassin's Creed 2].”
When asked if Haytham is in the game, Ashraf replied, “I don’t want to ruin any surprises, but Haytham is Edward’s son…”

“We’ve listened to our fans and brought back several elements from #AC2 and #ACB, like new hire groups like drunken pirates and we have new tools that will manipulate the crowd in fun ways.”

“You can stealth your way through about 85% of the game, but there are some forced battles, like boarding actions.”

Ashraf’s favorite part of making the game was “pushing seamless gameplay,” with his favorite assassin “biased towards Edward, but besides him, I’ll say Altaïr.”

“All of the sword takedowns will be unique (Hidden Blade too!).”

“Yes, we have third-person free aim.”

The most significant differences in Black Flag are “naval combat and exploration gameplay.”

The present day storyline picks up right after Assassin’s Creed III, “but now we’re inside Abstergo researching Edward Kenway because he does something important.”

“No double barrel pistols… maybe next time. ”

When asked if we could crouch, or if it’s area based like foliage cover Ashraf said, “It is foliage cover, but when you come out of the foliage you can remain crouched.”

“There isn’t currently a plan to do a game with Ezio’s daughter.”

“There is no character customization in the present day, but you can have a bit of fun with your Animus workstation.”

“The world is bigger [than Assassin's Creed III] and there are tons of cohesive side activities and side missions… more than any previous AC. All of the side mission/activities have something that they give you, like a tangible in-game *thing*.”

“Within the first 4 hours of the game, all of the side missions will be unlocked and the entire Caribbean world will be open to you to explore after about one hour.”

“Yes, there will be optional objectives, but they’re more about gameplay variation than difficulty.”

Plantations reset do reset so you can raid them again and “forts have side missions connected to them and become friendly, so they attack your enemies!”

“We have new factions that should be familiar to AC fans, with slight twists in their gameplay.”

“Edward’s tattoos are not customizable.”

“You always dual-wield either cutlasses or Hidden Blades, but you can quickly switch to a pistol in the middle of a combo.”

“We’ve always wanted to do a pirate game and there were no other games at handled it from a gritty, authentic perspective.”