I'm a bit disappointed with it too. I thought that this series would be an awesome time-travelling assassin thing, with the Desmond storyline being an excuse to go and visit a bunch of hugely different and interesting times, but instead we're stuck with an Ezio trilogy.
The Middle East, and then Renaissance Italy, both of which were fantastic. Then we got another Renaissance Italy game, and now we're getting a Renaissance period Ottoman empire. Thankfully that means that we'll get different architecture and culture, at least, but I was hoping for Feudal Japan, London, Paris, China, Ancient Rome - or just something different that you don't often see in video games.
Come to that, the Ottoman empire isn't half bad. It's cool, unique, it rarely shows up in video games or even popular culture, and it is very different. I guess I'm just bored of Ezio, and annoyed by Brotherhood.