I'd say it's more intelligence of knowing and utilizing body positioning and angles than strength, though I'm not saying he's week.
If you watch him long enough you will see he does make mistakes and turn the puck over and does get out muscled from time to time along the boards, just as any young defenseman would, BUT what sets Voynov apart from those other lesser blueliner's is his compete level and ability to recover for his mistakes.
He may turn it over, but his speed allows him to skate back faster than most, he won the faster skater in AHL 2 years back I think it was ? And he's smart enough to chose the right angles to force the play to the outside, use his body position to cut off stronger players from taking the puck to the front of the net, etc. etc.
It's like they say, can the Coach TRUST YOU to NOT HURT the Team ... Voynov may make mistakes, but he corrects himself in short order to where he rarely if ever hurts the team. Add to that his smarts at knowing when and how to pinch, or join the rush down low, and shot from the point on the PP ... his point production and goals will continue to be up there year after year as well. I hope Dean can work out a smart reasonable contract with Voynov that will keep him a King for many more years.
Voynov + Jeff Carter + 2012 Stanley Cup Champs
>>>> Jack Johnson + Whoever that 1st turns out to be
and I liked Jack