I think the best answer and if you could only pick one hockey trait, has to be: Bad hockey IQ. It leads to overhanding of the puck, not being able to get open for shots, not getting the puck to the right areas when he has it, horrible puck management decisions, being in the wrong spot when he doesn't have the puck... essentially everything wrong with Yak is related to how he thinks the game.
It's why he cant use his good-to-great stickhandling, skating, shot, and I'll even say forecheck (when he was rolling with Roy that one brief period he was a beast on the forecheck and in junior). All of which were accurately scouted when he went first overall.
Also, can we stop saying coaching yet? Look there's lots of players that had the exact same coaches as Yak in Edmonton and turned out well. Then Yak also gets Hitchcock this year and I guess Yeo (don't know how much time they've spent together). At some point, you have to stop blaming the people around him and just blame the player.
That being said, whoever takes a shot at him next (i'm sure he'll get another chance) would do well to put him with a quick, speedy center with good hockey IQ and some skill. He worked with someone like an overaged Derek Roy, he might work out with someone like Tyler Johnson.