Not disappointed at all.
I always wonder why do people dislike this Assassin's Creed idea to expand Ezio's story and to play with him in more than one game. Don't games usually have one protagonist anyway? What's so unusual about it? Assassin's Creed series gave us three protagonists for now. At first, I was a bit surprised that they'll continue to explore Ezio, but now after Brotherhood, I wouldn't have wanted it any other way. The Renaissance period is absolutely gorgeous and full of interesting individuals, events and places. It is not my favourite time period, it's not even on the top 5 list, but I continue to be amazed by how wonderfully it was done in the games and how much I started to love it since. Now I feel like I would be able to play ten more games in the same setting (also, the fact that I actually visited Venice, Florence and Rome turned it into an even better experience).
Besides, while Revelations is set in the same time period, it will be a very big change in the setting. Constantinople will give a very different vibe, and by being able to explore some other locations and Altair as well, I expect it to be nowhere near similar to Assassin's Creed II and Brotherhood. Same character? Yes. Same period? Yes. Same feeling? I doubt it.
About "not putting in much effort", I highly disagree. The amount of effort put to recreating cities such as Venice, Florence and Rome into an in-game equivalent from 500 years ago, and to do it so greatly, takes a lot of time, resources, investigating and learning. They didn't just invent random things and put it wherever they liked, they actually made sure everything was as historically correct as possible. I don't know how much time they spent on it, but it sure as hell looks like it was a lot. That's one of the reasons I like AC games so much.
Of course, I agree that it would be cool to explore other periods, but AC games are not primarily about exploring time periods; they have a set theme and story, and I'd rather have it if they finish the story as they planned it, instead of creating unrelated games that exist only for people to jump on roofs of some city in a time period they prefer. Hell yeah, I'd like to explore Ancient Greece or Victorian England, but I understand that the current story has no reason to go there, as of now. I'd like, however, for AC games to go somewhere else sometime in the future, but I'd also like for the current story to end with the current characters and to have the same theme and overall feel. No need to include more characters and time periods, it would just make a mess (and it would be quite strange for Desmond to have even more famous important assassin ancestors. Having two of them is a stretch already). As I understand, Revelations will finish Ezio's (and Altair's) story and we will be all set to move on to finish Desmond's story. Sure, it could've been done in less games and less extra material, but would it contain such a rich setting and world with its own rich and complex mythology, characters, places and events? Probably not. That's what makes it great (in the eyes of an obviously rabid fangirl, it seems).