[No spoilers]
Played close to 7 hours so far and I'm loving it. First of all, it sucks that they showed us in trailers and stuff pretty much what happen in the first 1/2 hours of the game (minus the Desmond part). The first thing that blowed my mind is constantinople, damn, the first time you arrive there, the colors, the architecture, the atmoshpere, how lively is the place, it's just the beautifulest world ever created for a game, and when the night falls, it's even better.
The game fells like Brotherhood, despite all the new additions to the game, so this is going to be a problem with some people and reviewers, but who cares, awesome is still awesome one year later.
From the new things so far, the assassin dens are the thing I'm enjoying more. The animus bits with Desmond are OK, nothing revolutionary but I has a cool thing to do inside it, the hookblade it's OK.
There is less collectibles now, some of them have an effect on the story of the game and are easy to spot because they are super bright.
Yusuf is awesome.
They removed the painting collectible and replaced with other thing that it's pretty cool if you love real world story.