The confidence/assertiveness I think is the biggest issue. He was so rattled by missing so many games with rookie year injuries, you could tell after that point he was playing too tentatively. He needed a sports psychologist between his rookie and sophomore seasons. And it's all because management insisted on playing him when he wasn't physically ready playing on his off position on a third line with Cody Eakin who had become a turnover machine and having to try to take the puck away from much bigger bottom 6 grinders.
Vegas didn't really do him and his mental state any favors making him ride the pressbox for huge stretches of the following season when he was on a two way deal and their AHL club was a 20-40 minute drive away. They could've kept him coming up to NHL practices while getting AHL starts, but instead he missed a ton of game time for seemingly no reason at all when he could have been playing top line minutes for the farm team and working on his pro game/recapturing his lost confidence.
By the time Vegas finally decided to let him get some hockey games in with the AHL team, he understandably looked defeated and unmotivated. Feels like it's been an uphill battle for him ever since. Like sure, on some level the drive just has to come from the player but the way Vegas treated his "development" was a bad joke. He had the vision, passing and hockey IQ to at least be a very serviceable second line center, or at least it seemed that way when he was a prospect. Before he finally got his NHL shot, he looked poised to have a very promising career and he looked worthy of the 6th overall selection.
Hell, my personal viewings, I thought Glass showed more promise than Suzuki who looked terrified of activating off the perimeter to get closer to traffic and I was not the least bit upset that Suzuki went to Montreal instead of Glass. Hindsight is obviously what it is, but I think Vegas really bungled his development. Hague, Whitecloud, and Thompson are really about the only prospects Vegas has developed properly.