Power will be picked higher based on his theoretical potential. Maybe he has it, and maybe he doesn't, but a 6'6 defenseman who can skate and is pretty good in all parts of the game is going to get tagged with having untapped potential.
I tend to believe that Power is a player whose hockey skills are pretty good, but not elite. I believe he does have untapped potential, and he'll get better defensively, offensively, skating, his physical play once he grows into his frame more and more. He has good hands and makes good decisions offensively. As he becomes more comfortable with the level of play, he'll take more offensive risks and as he gets stronger, his shooting power will become better. Defensively, he's not very physical, as has already been mentioned. Once he gets to his eventual prime playing weight, he'll be better able to take advantage of his size and reach. He'll be more effective in winning puck battles in the corners and better battling in front of the net.
Is he the most exciting player? Probably not, but neither is someone like Parayko. Neither is Ekblad. Neither is Jones. Neither is Pietrangelo. And even players like Hedman and Pronger that Power is compared to were not the most purely exciting or purely talented defensemen of their generation, but being 6'6 with good hockey sense and skating does matter. It will give you an advantage.