AC2, Brotherhood and Revelations weren't separated by dramatic changes, which is why they stuck with the AC2 label instead of moving onto 3 (and why some people feel calling the new one AC4 seems a little untimely). While the various locales have changed, it seems like buildings are also getting smaller with every iteration. To me, at least, the story seems lost in that we're trying to stop an event that has already occurred in real time (20121221), prophesied by aliens who were clearly the inspiration for the gods of old. But not much is being done with that aspect of it all, I feel, and we're instead getting lost in the stream of time, jumping from one character to the next while they forcefully insert something, anything, to make it relevant to the overarching story.
I kinda missed the AC bus, as it were, due to my complete and utter dislike of the first one, so I've always played the others without really connecting to them, and far later than everyone else. They aren't bad games, but they aren't great games, either. And then I'll read posts comparing its combat system to Arkham City's and I lose my shit because while the latter is almost perfect, the former is boring, and doesn't even make sense in the context of being an assassin. But whatever. That's just me.