To win the Cup, you have to maximize every dollar you have under that cap, and often frontload deals to get even more value than the normal cap limit. You can do a lot of things well, make a lot of great moves to improve your hockey club, but if you make just one or two mistakes where you don't maximize every dollar you have, it will come back to bite you, and cost you the Cup.
Ken Holland is considered the best GM in the league, and he's done so much well over the years, but the last two years, just a few mistakes (trading Leino, some of the depth signings he made, sitting on his laurels a bit too much), and all that other great work goes for naught. Now the Red Wings have gone two years without making the final four.
The Blackhawks: They maximized every dollar last year, even going way over the cap with their bonuses, and they won the Cup because they crammed so much talent and so much value under that cap. But this year, despite that great core they still have, all the talent they have, the few mistakes Dave Tallen made caught up with them (the Byfuglien and Versteeg contracts came back to bite them), and then the new GM, Stan Bowman, didn't make good decisions in reloading the team for this season. As a result, they struggled all season, and were eliminated in the first round.
The San Jose Sharks: Doug Wilson drafted Logan Couture, Ryane Clowe, etc, he brought in Thornton, Boyle, and Heatley, all on lopsided trades... He did a lot of good, but the Wallin and Huskins contracts, and having the 2 million dollar backup in Niittymaki this season, those few mistakes undid everything else, because that precious cap space he wasted on mistakes was the difference between having another top 3 defenseman, and not having one. That's why the Sharks are close every year, but they never make it. The core of players Doug Wilson has brought in are good enough to bring that team fairly close every year, but then when it comes time for Doug Wilson to close the deal, and put the finishing touches on his roster, he fails. Instead of going out and signing the top 4 defenseman they need, and making smart signings for the lower lines, he makes head-scratching additions like the Wallin and Huskins ones, Jamal Mayers, and so on. And that's the difference. You put Wallin and Huskins on the Canucks instead of Hamhuis (I know it doesn't balance out $ wise perfectly, but for the sake of discussion...), and put Hamhuis on the Sharks, and then you swap Jamal Mayers and Scott Nichol for Raffi Torres and Maxim Lapierre, and the Sharks probably beat the Canucks. But it was Mike Gillis last offseason, when the Sharks and Canucks were both in very similar situations, who (Ballard aside), made smart final-touch additions, whereas Doug Wilson made bad additions. And that was the difference between the two teams. Doug Wilson's mistakes cost his team a real chance at the Cup, again.
Those are just a few examples . There are countless more. As for the Vancouver Canuck's example, obviously it's Keith Ballard. The Sedins are on great contracts, so are Kesler and Burrows, and many others. The Higgins and Lapierre acquisitions were excellent by Mike Gillis as well. However, despite all the good, that one mistake comes back to bite you almost every time. When you have a 4.2 million blueliner sitting out, what that means is you're basically playing with a roster that's 4(.2) million under the cap. No none-cap team has won the Cup in years. The basic concept is you can't beat a team with 4 million more in talent than you. You have to maximize every cent. And with 4.2 million sitting on the sidelines, the Canucks did not.
And the other real tragedy of the Ballard deal is, isn't this the exact scenario Gillis outlined when defending the acquisition? The media tried to get on him about how it was a bad trade, but he stubbornly defended it, saying it would all be worth it in the end, because he wanted another defenseman on his roster come playoff time. In his mind, it didn't matter that he overpaid, because he just wanted another defenseman, plain and simple, no matter what, and he was willing to overpay because he saw it as being that important. He knew defensemen get injured in the playoffs a lot, and he wanted to have that spare around, no matter the cost.
Well, everything has gone accordingly. We are very deep in the playoffs, and the Canucks have major injuries on defense. This was the whole point of overpaying for Ballard, for this exact circumstance, so the Canucks would have that quality spare. Yet, he isn't even playing, not even in the this worst case injury scenario. It's Andrew Alberts, making 1/4 of his salary, who is playing instead, along with the rookie, Chris Tanev.
So that's what I'm taking from all this. You have to maximize every cent. The Sedins are being neutralized by Chara, Kesler is injured, Burrows still looks great, but he's having to play against Chara too. The Canucks still have Lapierre, Torres, Hansen, Higgins, etc, but these are secondary players. Whereas on the other side of it, the Bruins have Krejci, Bergeron, Lucic, Horton before he got injured, Marchand, Seguin, Ryder, Recchi is playing well, Peverley was playing in the top 6 in Atlanta, and playing well, and he's on their 4th line... They just have so many horses, and so much depth. The Canucks are so reliant on just 3 forwards to carry their team, whereas boston just has so many more top 6 forwards. They may not have as many top 3 forwards, but they have way more horses. And that was the one thing that worried me to start the season, and earlier in the playoffs, if you look at my comments (before the Sharks series I mentioned, I believe), that the Canucks sacrificed offensive forward depth for their defense last offseason. And that's where not maximizing every dollar really hurts. The Canucks could so badly use that 4th horse forward on the ice with to go along with the Sedins and Kesler, but instead they have Keith Ballard sitting in the press box.
It's just one mistake against many many good moves, but it's that one mistake which can be the difference. The Bruins brass may not be perfect either, but their biggest mistake this season, the Kablerle trade, is something that may hurt them a few years down the row because of the assets they gave up. But, in terms of maximizing their dollar amount this season, the Kaberle trade didn't hurt them at all. They got him at the deadline's reduced cap hit, so their Kaberle mistake is much different than the Canuck's Ballard mistake for that reason alone, not to mention the fact Kaberle is playing, and is more than serviceable as a bottom pairing defenseman.
No one is above having mistakes come back to bite them. Even the supposedly untouchable great GM Ken Holland, he made just a few mistakes after a long succession of great moves, and they showed up immediately in his team's results: two years without making the final four. No one is above it, and Gillis' one glaring mistake just cost his team in the worst way.