Skating, skating, skating. Just watched that video, and while I hate to be the downer, I can immediately see why he fell so far. His skating isn't just lacking, it's quite poor. He can keep up with play while going forward only by putting in twice the effort everyone else is. He's got really big, choppy, inefficient strides, can't seem to turn well (or at all, really) in tight spaces, and lacks the quick step necessary to accelerate out of tight spaces, even going forward. Lateral acceleration to get out of a tight spot seems to be a 'forget about it'.
He does seem to have a decent shot and vision, and a decent sense for positioning, but he's never going to be able to keep up at the NHL level. With a lot of work on his technique, he could improve his skating, but from what I can see watching that video, he has a very long way to go, and even a substantial improvement likely won't be enough to make it in a league as fast and agile as the NHL is today.
Skating is a hard skill to teach at a high level, and probably the most crucial one at the NHL level these days. Even in the 4th round, I don't think this was a good pick.