Like others have mentioned, he's still pretty young for a d-man and has loads of time to develope into the type of player he was projected to be.
There is alot of precedence for this in the NHL. Look at Chara, Jovo, McCabe, Pronger, Foote. I don't know if he'll be as good as any of them, but they all needed several seasons at the NHL level to start realizing their potential. NHL Dmen only start hitting their prime at 30.
I think that the fact that we have Downey in the line up help him too since when he was playing physical at the beginning of the year, guys like breshear and domi wanted to fight him and it's not his job to do that and i think that with downey in the line up, he knows he can count on him if something happen.