If given room, when he has the puck he is great at starting rushes and can make pretty good stretch passes, and can also carry the puck deep into the o-zone if need be.
However, he is absolutely terrible when under pressure in the defensive zone, which is clearly not a good trait if you're an NHL defenseman. Way too many giveaways and missed assignments. This unfortunately negates his positive traits and often makes him an overall liability, IMO.
The Leafs are going to rely on him to continue to play big minutes, which could spell disaster come playoff time. If you are prone to continual defensive zone blunders when pressured, I don't care how good you are at skating and passing, you are a huge risk to your team, especially when playing such big minutes against quality teams.