I'd say promoting Dubas to GM. While he did learn the ropes over time, eventually became a solid GM, he inherited an absolutely unbelievable situation (incredible young talent and tonnes of cap flexibility), and made a series of brutal mistakes in his first 12 months as GM that simply destroyed our ability to truly contend.
The really key mistakes:
- Signed Tavares to a 7x$11 mil deal, to be our 2C. When we already had a quality 2C in Kadri, who was locked down for a number more years at $4.5 mil
- Didn't remotely fit team needs
- Dubas misread his game. Slow skating and weak defensive play means he's just a very mediocre even strength player, despite elite puck skill
- Ruined our "internal cap structure" - JT is simply not that good, Matthews and Marner could clearly see they were better, demanded JT+ $$. The teams with great cap management get star(s) signed to discounts, and then everyone else falls in line salary wise relative to that
- Had all the leverage in the Matthews deal, Matthews was an RFA, Dubas could afford to be patient. But with TONNES of time left to negotiate, utterly caved on both AAV and term - $11.64 AAV made him the 2nd highest paid centre in the entire league (behind only McDavid), and 5 years bought only a single UFA season. Insanely terrible negotiating
- Same thing with the Marner deal, 6x$10.9. Bought just 2 UFA seasons, and made him the 2nd highest paid winger in the league (behind only Panarin). Again signed this with tonnes of negotiating time left
The Leafs had this incredible young core, that was crazy stacked with young forwards - Matthews, Marner, Nylander, Kadri, Hyman, Brown, Moore, Kapanen, Johnsson, Engvall, etc. The clear focus had to be:
- Negotiating hard with the many RFAs
- Improve the clearly weak defence, and somewhat weak goaltending (with Andersen starting to struggle shortly after Dubas took over)
- Switching out some of those more "skill forwards" for some grit - had great talent at forward, but not the best grit/skill mix
Instead he just f***ed our cap with the Tavares, Matthews and Marner contracts, and that actually lead to our 4 grittiest forwards becoming cap casualties. Hyman and Brown very direct cap casualties, and Kadri and Moore more indirect, traded for dmen in attempts to improve the D on a shoestring budget, where no dman could make much $$.