The lion's share of the game will take place in Constantinople (or, as it's known today, Istanbul) separated into four districts: Constantin, Beyazid, Imperial, and Galata. If you want a change of scenery (fairy chimneys!) some of the game will also take place in Cappadocia. Inside the cities, you'll be able to establish Assassin's Dens, a sort of expanded Borgia Tower mechanic from Brotherhood. Complete various scenarios, take down the regional Templar presence, establish a Den, and then "upgrade the buildings in that region along with adding ziplines across the rooftops."
But it wouldn't be a video game if you couldn't use that hookblade to mess dudes up. Lucky for you it can be used to pull said dudes in for a mean combo. More interested in bombs, you say? So is Ezio! He can now make up to 300 bombs using a crafting system. Along with the new toys comes a new level cap and a more narrative-focused multiplayer mode with the ability to customize your character and create guilds.
Lastly, Ubisoft Montreal is using a new facial capture technology called Mocam which Game Informer calls an "intriguing amalgamation of traditional animation, performance capture and the fascinating new style of performance on display in games like LA Noire." And yes, Ubi is doing all that with another one-year turnaround. With the success of Brotherhood, we're certainly giving it the benefit of the doubt.
Of course, you can find more details (and pretty pictures!) in the June issue of Game Informer.