For me, I found that the lack of direction led me to explore the world more. Even though the story is linear, the world is huge and a lot if things are spread here and there. You will be rewarded for exploration.
I found the stealth mechanics to be on par if not better than Batman Arkham series. Different strokes I guess.
As for the authenticity of Boston, I don't know how much I can hold that against the game. I'm really looking forward to Ubisoft's Watchdogs mainly because Chicago is my home town and I really want to explore it. I guess it would be a downer if its not accurate to some extent.