I can't comment too much on the 80's Devils since that was way before my time and I sort of write off that part of franchise history. But since someone mentioned the Devils letting go of Lou as the worst decision, I want to point out what the worst decisions I think are for Devils post 1988 history, and you'll see most of them were made during Lou's last 4 years. And they all come from what is #1:
1. Not giving up the 2012 1st rounder - this right here is an absolute firable offence that had a horrible spiral that led to the Devils turning out what is the worst case scenario of a team in the NHL: an old team that is fairly bad, but not bad enough to finish bottom-3. And for 2014, this was a bad team without a first-rounder. Some of the following are a direct result of it:
2. Trading the 9th for Cory Schneider - aka the anti-tank commander. This goes up the list purely because he was so good when we didn't need it (the McEichel and Matthews drafts) and then became terrible after his extension.
3. Drafting Pavel Zacha over pretty much anyone else in the 2015 draft
4. The moves made during the 2013-14 season trying to foolishly compete Signing Brunner, Ryder, Ryan Clowe, etc. Trading for Tuomo Ruutu. This is all because the Devils were going into that season with no 1st round pick due to #1. Funny enough, that team could have made the playoffs if they weren't epically bad in the shootout, but they would've been easy first round fodder.
5. Keeping David Conte (the scouting director) employed purely because of what he did in the 90's.
Other than that, I don't know. Not bringing back Nieuwendyk was a mistake for the 2003/04 team. But Lou was genuinely a genius for about 25 years. I'm actually pretty happy for him succeeding elsewhere, as it shows that he truly is a HoF talent GM. But yeah, those last few years with the Devils were rough.