Honestly, I have no problem with this at all. There will be more than enough future Assassin's Creed games set in other times, we can wait.
Until then I'm more than happy to have a story that will have Ezio scouring foreign lands for artifacts that Altair perhaps died trying to find. There is a lot of potential in what they're doing, and after the previous three games I have faith that they will make use of it.
As far as the story goes I actually think this one could really take it up a notch. As far as gameplay goes, I'm also pretty excited. I mean come on, we'll have Altair on his quest to Constantinople, probably to find the pieces of Eden that he had seen on the 3D map. Think of the cool new gadgets and perhaps new gameplay mechanics you can expect as a result of this. Same thing with Ezio, just in a different time period.
There IS a lot of potential, and unlike so many other game series that follow a single protagonist, Assassin's Creed has a history of doing character development RIGHT. I can pretty much guarantee that if you liked the previous installments, you'll like this one as well.