In addition to a pretty intense ship-to-ship battle that Ubisoft premiered during their E3 press conference, the company showed a brand new Assassin's Creed 3 demo on the E3 show floor. Set in Boston during the summer of 1773, the level demonstrates some of the new stealth mechanics being introduced in this entry.
Connor, AC3's hero, is able to discover randomly-generated quests scattered throughout the world. They're optional and usually involve saving locals from ne'redowells. In this case, a woman wanted someone to save her friend from being tortured in the stocks. The friend was located in a small courtyard, surrounded by buildings on all sides. In the middle of the courtyard was a large tree and a few bushes.
Connor was able to press up against the wall in the alleyway leading to the courtyard (using a new addition, corner cover, to peek around a wall). Taking out a guard with a quick assassination, Connor moved towards the bushes, which act as ground-based haystacks, rendering him invisible so long as no one pokes their head in.
A moment later and Connor was climbing the lone tree, using the same technqiues that we've seen in the frontier sections of Assassin's Creed 3. Edging out onto a limb, Connor took out another guard, using a Chinese throwing rope to latch onto an unfortunate bastard standing underneath a bough. The final guard was quickly dispatched with a leaping hidden blade attack, and the man was quickly saved. That man...would grow up to be Abe Lincoln! Actually no, that's false, but wouldn't that be cool?
The other decidedly cool new stealth element is auto-blending. You used to have to enter pre-set crowds of people in order to blend into an environment. Now whenever Connor is standing near at least two people, any two people, he'll blend in. You're notified of his visibility with lines coming from above his head, showing how many people you're surrounded by. It's just another way Assassin's Creed 3 makes stealth look and feel more natural than it's been in previous games.
And finally, while it's not stealthy at all, it's worth calling your attention to one of the best assassinations we've seen in the franchise so far. If two guards are standing next to each other and you've got a bayonetted musket equipped, you can stab the first guard and then blast the second-guard in a lovely one-two punch. More of this, please.