It's 4 things.
1) Special teams.
When you look at the top power play units over the last 5 years, they have had big right handed shots on the left half wall. Think Ovechkin, Stamkos, Pastranak, where they tee up on the left side of the power play above circle with that right shot. Matthews is a left handed shot, so he doesn't get as much of a clean look. Marner is a right handed shot but he doesn't have a very good shot. If only they were reversed. We need to address this somehow, as obviously this isn't the only formula to have power play success, just one of them. Matthews is never going to score 100 points if he can't score regular goals on the P.P... he also has a tendency to give the puck away on the player as well. A top 5 P.P in the league vs a top 20 P.P will result in a substantial difference in the standings, P.K as well.
2) Mindset, standards and consistency.
This team lacks the elite mental drive and willingness to do everything it takes to win. They are going at it at about a 90 % level. It's clear they haven't reached there potential. They don't have the mental drive of players playing for there lives or players that have the drive to win the whole thing. The fact William Nylander was willing to sit out an entire season shows you where his mind was. We've had less talented teams work much harder. This team legitimately does try to get by on there skill and it's an issue. Some blame the coach, same blame the GM, some blame the players. I don't know who or whom is responsible but this does seem to be an issue.
3) Lacking a top 10 defence group in the NHL.
We don't have a top 10 RHD shutdown defensmen. I'm nit picking here, I actually think Ceci is pretty decent and Barrie is being misused a little bit. Muzzin is a great piece, i'm very happy with him. Rielly isn't a top 10 d-man in the league either. This isn't a knock on Rielly by the way, the fact we have a top 5 offensive d-man in the league is great... however his defensive positioning on 2 on 1's is atrocious. Dermott for example is better at this as a sophmore. Rielly also makes some pretty boneheaded plays on the P.P, continuously throwing the puck into traffic on the power play. It's a very lower percentage shot, and giving the puck away on the player can kill 30 seconds of a 2:00 minute power play, and possibly the entire power play if we can get into the zone again. If we had a Parayko type defensmen it would make a huge difference. If our best d-man is going to be in the offensive mold, we need a top shutdown defender to play with him if he's going to log 25 minutes a night.
4) Overpaying our personnel.
We live in a cap world where every decision is magnified. Overpaying 3-4 players is going to make it extremely difficult moving forward. It's not just difficult to fill out the team it's just wrong.
These are 4 very big issues. I'm glad I don't have to fix them. It seems all 3 of GM, Coach and players are to blame.