What is reasonable to expect from him?
I find Scotty Bowman's old line about every rookie in the lineup costing you x number of games (I can't remember what the number way) is generally accurate. High-value rookies will help you win, but they will also, because they are rookies, make bonehead plays that cost you games. There's no way around it, and you hope that your team is significantly more talented with that rookie in the lineup than the vet who might have replaced him. I was thinking about this on Sunday when Atlanta took out the Seahawks. Russell Wilson made a rookie mistake at the end of the first half that cost the Hawks a field goal attempt. The Seahawks ultimately lost the game by less than a field goal. You can look at that one moment and say, yes, there are all sorts of other things that might have gone differently if other players had been better, but that's a play where a typical rookie mistake is easy to isolate for its damage to Wilson's chances of winning.
These will happen to Hamilton. He'll miss a coverage, pull a Wideman trying to get back on a two on one, make a terrible pass that puts the puck on Crosby's stick in the slot. It's going to happen mid-way through the third in a tied game, result in a goal and a spot in the doghouse, and cost the Bruins a game. Be prepared for it because there's nothing you can do about it. He's a rookie.
I think his overall impact on the Bruins will be more significant for the pressure it puts on the opposing D - he can make better long passes than most of the Bruin D, and, if he isn't scared into submission by Julien, he has the instinct and the stride to appear in the rush unexpectedly. Just the need to account for what he can do will open up space, and that will make a difference even if it isn't immediately obvious that it's Hamilton that is making it happen.