I've played with him, his speed in a straight line is average, but his quickness is unbelievable. Like if you had him race someone else in the Q up and down the ice, it would be very close... But if you had them zig zag between pylons he would beat the other guy flat out.
He has good shot accuracy and really good hand eye. His passing is solid and he has silky hands. His toedrag is unbelievable.
His defense is lacking, he seems a bit slow on defense and on draws. But in the offensive zone he is a constant threat. He has a very high intensity level and works like a dog, he wants the puck more than anyone on the ice. He has good hockey IQ too, maybe not as high as McDavids but its still good!
If he works on his defense he will come out as the best pick of the draft. IMO if he was 6' or 6'1 he would have gotten picked first over Roy.
He dropped the mitts with Roy and absolutely man-handled him even though Roy is 6" taller than him!
Comparing him to Nate MacKinnon is a good comparison. The 2015 NHL draft will be a deep draft in the forward category. If you want another comparison I would probably compare him to a Brendan Gallagher with less defense and more offense.